java.util示例代码大全-转载

在Javaeye上看到一篇贴子，还不错，都是Java编码中比较实用的代码。也可以直接去Java参考中文站去阅读。

java.util
e331. 产生一个随机数

Random rand = new Random();
   
    // Random integers
    int i = rand.nextInt();
    // Continually call nextInt() for more random integers ...
   
    // Random integers that range from from 0 to n
    int n = 10;
    i = rand.nextInt(n+1);
   
    // Random bytes
    byte[] bytes = new byte[5];
    rand.nextBytes(bytes);
   
    // Other primitive types
    boolean b = rand.nextBoolean();
    long l = rand.nextLong();
    float f = rand.nextFloat();     // 0.0 <= f < 1.0
    double d = rand.nextDouble();   // 0.0 <= d < 1.0
   
   
    // Create two random number generators with the same seed
    long seed = rand.nextLong();
    rand = new Random(seed);
    Random rand2 = new Random(seed);

e332. 将一个字符串分解成字

String aString = "word1 word2 word3";
    StringTokenizer parser = new StringTokenizer(aString);
    while (parser.hasMoreTokens()) {
        processWord(parser.nextToken());
    }

e333. 建立一个定制事件

A new custom event must extends EventObject. Moreover, an event listener interface must be declared to allow objects to receive the new custom event. All listeners must extend from EventListener.

This example demonstrates all the steps necessary to create a new custom event.

// Declare the event. It must extend EventObject.
    public class MyEvent extends EventObject {
        public MyEvent(Object source) {
            super(source);
        }
    }
    
    // Declare the listener class. It must extend EventListener.
    // A class must implement this interface to get MyEvents.
    public interface MyEventListener extends EventListener {
        public void myEventOccurred(MyEvent evt);
    }
    
    // Add the event registration and notification code to a class.
    public class MyClass {
        // Create the listener list
        protected javax.swing.event.EventListenerList listenerList =
            new javax.swing.event.EventListenerList();
    
        // This methods allows classes to register for MyEvents
        public void addMyEventListener(MyEventListener listener) {
            listenerList.add(MyEventListener.class, listener);
        }
    
        // This methods allows classes to unregister for MyEvents
        public void removeMyEventListener(MyEventListener listener) {
            listenerList.remove(MyEventListener.class, listener);
        }
    
        // This private class is used to fire MyEvents
        void fireMyEvent(MyEvent evt) {
            Object[] listeners = listenerList.getListenerList();
            // Each listener occupies two elements - the first is the listener class
            // and the second is the listener instance
            for (int i=0; i<listeners.length; i+=2) {
                if (listeners[i]==MyEventListener.class) {
                    ((MyEventListener)listeners[i+1]).myEventOccurred(evt);
                }
            }
        }
    }

Here's an example of how to register for MyEvents.

MyClass c = new MyClass();
    
    // Register for MyEvents from c
    c.addMyEventListener(new MyEventListener() {
        public void myEventOccurred(MyEvent evt) {
            // MyEvent was fired
        }
    });
e334. 实现一个简单的事件通知器

The Observer and Observable classes are superseded by a more elaborate event framework (see e333 建立一个定制事件). However, these two classes can still be useful for implementing a simple event notifier. 
// Declare the model
    class MyModel extends Observable {
        // The setChanged() protected method must overridden to make it public
        public synchronized void setChanged() {
            super.setChanged();
        }
    }
// Create the model
    MyModel model = new MyModel();
    
    // Register for events
    model.addObserver(new Observer() {
        public void update(Observable o, Object arg) {
        }
    });
    
    // Indicate that the model has changed
    model.setChanged();
    
    // Fire an event to all the views
    Object arg = "some information about the event";
    model.notifyObservers(arg);


e335. 列出所有可用的地区

Locale[] locales = Locale.getAvailableLocales();
    
    for (int i=0; i<locales.length; i++) {
        // Get the 2-letter language code
        String language = locales[i].getLanguage();
    
        // Get the 2-letter country code; may be equal to ""
        String country = locales[i].getCountry();
    
        // Get localized name suitable for display to the user
        String locName = locales[i].getDisplayName();
    }
Here's a sample of output using a default locale of Locale.ENGLISH: 
Language Code, Country Code, Localized Name
    ar, , Arabic
    ar, AE, Arabic (United Arab Emirates)
    ar, BH, Arabic (Bahrain)
    ar, DZ, Arabic (Algeria)
    ar, EG, Arabic (Egypt)
    ar, IQ, Arabic (Iraq)
    ar, JO, Arabic (Jordan)
    ar, KW, Arabic (Kuwait)
    ar, LB, Arabic (Lebanon)
Here's a sample of output using a default locale of Locale.FRENCH:
Language Code, Country Code, Localized Name
    ar, , arabe
    ar, AE, arabe (Emirats Arabes Unis)
    ar, EG, arabe (Egypte)
    ar, IQ, arabe (Irak)
    ar, JO, arabe (Jordanie)
    ar, KW, arabe (Koweit)
    ar, LB, arabe (Liban)


e336. 设置默认地区

There are two ways to change the default locale. The first is to set it on the command line: 
> java -Duser.language=2-char-language-code -Duser.region=2-char-country-code MyApp
    
    // Set only language code
    > java -Duser.language=fr -Duser.region= MyApp
    // Set language and country code
    > java -Duser.language=fr -Duser.region=CA MyApp
The second way to change the default locale is to call Locale.setDefault():
// Get default locale
    Locale locale = Locale.getDefault();
    
    // Set the default locale to pre-defined locale
    Locale.setDefault(Locale.FRENCH);
    
    // Set the default locale to custom locale
    locale = new Locale("fr", "CA");
    Locale.setDefault(locale);


e337. 将一个值与一个对象联系起来

This example demonstrates how to associate a value with an arbitrary object. The technique involves saving the object and the associated value as a key/value pair in an IdentityHashMap. A HashMap cannot be used for this purpose since if two objects happen to equal via the Object.equals() method, one of the objects will not be stored. 
// Create the map
    Map objMap = new IdentityHashMap();
    
    // Add the object and value pair to the map
    Object o1 = new Integer(123);
    Object o2 = new Integer(123);
    objMap.put(o1, "first");
    objMap.put(o2, "second");
    
    // Retrieve the value associated with the objects
    Object v1 = objMap.get(o1);     // first
    Object v2 = objMap.get(o2);     // second

Arrays
--------------------------------------------------------------------------------
e338. 数组比较

// null arrays are equal
    boolean[] bArr1 = null;
    boolean[] bArr2 = null;
    boolean b = Arrays.equals(bArr1, bArr2);                   // true
   
    // Compare two boolean arrays
    bArr1 = new boolean[]{true, false};
    bArr2 = new boolean[]{true, false};
    b = Arrays.equals(bArr1, null);                            // false
    b = Arrays.equals(bArr1, bArr2);                           // true
   
    // There are equals() methods for all eight primitive types
    b = Arrays.equals(new byte[]{0}, new byte[]{0});           // true
    b = Arrays.equals(new char[]{'a'}, new char[]{'a'});       // true
    b = Arrays.equals(new short[]{0}, new short[]{0});         // true
    b = Arrays.equals(new int[]{0}, new int[]{0});             // true
    b = Arrays.equals(new long[]{0L}, new long[]{0L});         // true
    b = Arrays.equals(new float[]{0F}, new float[]{0F});       // true
    b = Arrays.equals(new double[]{0D}, new double[]{0D});     // true
   
    // When comparing Object arrays, null elements are equal.
    // If the elements are not null, Object.equals() is used.
    b = Arrays.equals(new String[]{"a"}, new String[]{"a"});   // true
    b = Arrays.equals(new String[]{null}, new String[]{null}); // true
    b = Arrays.equals(new String[]{"a"}, new String[]{null});  // false

e1074. 装填数组

The Arrays class has conveninent methods for filling arrays of all eight primitive types:

boolean[] booleanArr = new boolean[10];
    boolean booleanFillValue = false;
    Arrays.fill(booleanArr, booleanFillValue);
    
    byte[] byteArr = new byte[10];
    byte byteFillValue = (byte)0xFF;
    Arrays.fill(byteArr, byteFillValue);
    
    char[] charArr = new char[10];
    char charFillValue = 0xFF;
    Arrays.fill(charArr, charFillValue);
    
    short[] shortArr = new short[10];
    short shortFillValue = 0xFF;
    Arrays.fill(shortArr, shortFillValue);
    
    int[] intArr = new int[10];
    int intFillValue = -1;
    Arrays.fill(intArr, intFillValue);
    
    long[] longArr = new long[10];
    long longFillValue = -1;
    Arrays.fill(longArr, longFillValue);
    
    float[] floatArr = new float[10];
    float floatFillValue = -1;
    Arrays.fill(floatArr, floatFillValue);
    
    double[] doubleArr = new double[10];
    double doubleFillValue = -1;
    Arrays.fill(doubleArr, doubleFillValue);

There is also a method for filling object arrays:

String[] StringArr = new String[1];
    String StringFillValue = "";
    Arrays.fill(StringArr, StringFillValue);

By default, the nine fill methods shown above set all the elements in the array with the fill value. Each fill method has an overloaded version that can restrict the fill to a contiguous range of elements:

// Fill elements 0, 1, 2, and 3; the end index is exclusive
    int startIndex = 0;
    int endIndex = 4;
    
    Arrays.fill(booleanArr, startIndex, endIndex, booleanFillValue);
    Arrays.fill(byteArr, startIndex, endIndex, byteFillValue);
    Arrays.fill(charArr, startIndex, endIndex, charFillValue);
    Arrays.fill(shortArr, startIndex, endIndex, shortFillValue);
    Arrays.fill(intArr, startIndex, endIndex, intFillValue);
    Arrays.fill(longArr, startIndex, endIndex, longFillValue);
    Arrays.fill(floatArr, startIndex, endIndex, floatFillValue);
    Arrays.fill(doubleArr, startIndex, endIndex, doubleFillValue);
    Arrays.fill(StringArr, startIndex, endIndex, StringFillValue);
e339. 打乱一个列表或数组的元素

Use Collections.shuffle() to randomly reorder the elements in a list. 
// Create a list
    List list = new ArrayList();
    
    // Add elements to list
    
    // Shuffle the elements in the list
    Collections.shuffle(list);
    
    // Create an array
    String[] array = new String[]{"a", "b", "c"};
    
    // Shuffle the elements in the array
    Collections.shuffle(Arrays.asList(array));


e340. 将一个收集器装换成一个数组

// Create an array containing the elements in a list
    Object[] objectArray = list.toArray();
    MyClass[] array = (MyClass[])list.toArray(new MyClass[list.size()]);
    
    // Create an array containing the elements in a set
    objectArray = set.toArray();
    array = (MyClass[])set.toArray(new MyClass[set.size()]);
    
    // Create an array containing the keys in a map
    objectArray = map.keySet().toArray();
    array = (MyClass[])map.keySet().toArray(new MyClass[set.size()]);
    
    // Create an array containing the values in a map
    objectArray = map.values().toArray();
    array = (MyClass[])map.values().toArray(new MyClass[set.size()]);

e341. 将一个数组转换成一个收集器

// Fixed-size list
    List list = Arrays.asList(array);
    
    // Growable list
    list = new LinkedList(Arrays.asList(array));
    
    // Duplicate elements are discarded
    Set set = new HashSet(Arrays.asList(array));

Collections
--------------------------------------------------------------------------------
e342. 实现一个队列

LinkedList queue = new LinkedList();
   
    // Add to end of queue
    queue.add(object);
   
    // Get head of queue
    Object o = queue.removeFirst();
   
    // If the queue is to be used by multiple threads,
    // the queue must be wrapped with code to synchronize the methods
    queue = (LinkedList)Collections.synchronizedList(queue);

e343. 实现一个栈

LinkedList stack = new LinkedList();
   
    // Push on top of stack
    stack.addFirst(object);
   
    // Pop off top of stack
    Object o = stack.getFirst();
   
    // If the queue is to be used by multiple threads,
    // the queue must be wrapped with code to synchronize the methods
    stack = (LinkedList)Collections.synchronizedList(stack);

e344. 实现一个最近最少使用的（LRU）高速缓存

// Create cache
    final int MAX_ENTRIES = 100;
    Map cache = new LinkedHashMap(MAX_ENTRIES+1, .75F, true) {
        // This method is called just after a new entry has been added
        public boolean removeEldestEntry(Map.Entry eldest) {
            return size() > MAX_ENTRIES;
        }
    };
   
    // Add to cache
    Object key = "key";
    cache.put(key, object);
   
    // Get object
    Object o = cache.get(key);
    if (o == null && !cache.containsKey(key)) {
        // Object not in cache. If null is not a possible value in the cache,
        // the call to cache.contains(key) is not needed
    }
   
    // If the cache is to be used by multiple threads,
    // the cache must be wrapped with code to synchronize the methods
    cache = (Map)Collections.synchronizedMap(cache);
e345. 遍历一个收集器中的元素

This example demonstrates how to iterate over the elements of various types of collections.

// For a set or list
    for (Iterator it=collection.iterator(); it.hasNext(); ) {
        Object element = it.next();
    }
    
    // For keys of a map
    for (Iterator it=map.keySet().iterator(); it.hasNext(); ) {
        Object key = it.next();
    }
    
    // For values of a map
    for (Iterator it=map.values().iterator(); it.hasNext(); ) {
        Object value = it.next();
    }
    
    // For both the keys and values of a map
    for (Iterator it=map.entrySet().iterator(); it.hasNext(); ) {
        Map.Entry entry = (Map.Entry)it.next();
        Object key = entry.getKey();
        Object value = entry.getValue();
    }

e346. 用收集器存储基本类型

Collections can only store objects, not primitive types like int and double. Primitive types must be placed in a wrapper object before they can be placed in a collection. This example demonstrates the storing of int values in a Map.

See also: e58 包装基本类型为对象.

// Create map
    Map map = new HashMap();
    
    // Create int wrapper object
    Integer refInt = new Integer(123);
    
    // Store int in map
    map.put("key", refInt);
    
    // Get int value from map
    refInt = (Integer)map.get("key");
    
    // Get the integer value from wrapper object
    int i = refInt.intValue();

e347. 建立一个收集器的副本

These examples create a shallow copy of a collection. That is, the new collection contains references to same objects as the source collection; the objects are not cloned.

List stuff = Arrays.asList(new String[]{"a", "b"});
    
    // Make a copy of a list
    List list = new ArrayList(stuff);
    List list2 = new LinkedList(list);
    
    // Make a copy of a set
    Set set = new HashSet(stuff);
    Set set2 = new TreeSet(set);
    
    // Make a copy of a map
    Map map = new HashMap();
    // Add key/value pairs ...
    Map map2 = new TreeMap(map);

e348. 将一个收集器设为只读的

将一个收集器设为只读的 involves wrapping the collection in another object whose mutation methods all throw UnsupportedOperationException.

List stuff = Arrays.asList(new String[]{"a", "b"});
    
    // Make a list read-only
    List list = new ArrayList(stuff);
    list = Collections.unmodifiableList(list);
    
    try {
        // Try modifying the list
        list.set(0, "new value");
    } catch (UnsupportedOperationException e) {
        // Can't modify
    }
    
    // Make a set read-only
    Set set = new HashSet(stuff);
    set = Collections.unmodifiableSet(set);
    
    // Make a map read-only
    Map map = new HashMap();
    // Add key/value pairs ...
    map = Collections.unmodifiableMap(map);

Lists
--------------------------------------------------------------------------------
e349. 建立一个列表

// Create the list
    List list = new LinkedList();    // Doubly-linked list
    list = new ArrayList();          // List implemented as growable array
   
    // Append an element to the list
    list.add("a");
   
    // Insert an element at the head of the list
    list.add(0, "b");
   
    // Get the number of elements in the list
    int size = list.size();          // 2
   
    // Retrieving the element at the end of the list
    Object element = list.get(list.size()-1);   // a
   
    // Retrieving the element at the head of the list
    element = list.get(0);                      // b
   
    // Remove the first occurrence of an element
    boolean b = list.remove("b");      // true
    b = list.remove("b");              // false
   
    // Remove the element at a particular index
    element = list.remove(0);          // a

e350. 将一个列表排序

// Create a list
    String[] strArray = new String[] {"z", "a", "C"};
    List list = Arrays.asList(strArray);
   
    // Sort
    Collections.sort(list);
    // C, a, z
   
    // Case-insensitive sort
    Collections.sort(list, String.CASE_INSENSITIVE_ORDER);
    // a, C, z
   
    // Reverse-order sort
    Collections.sort(list, Collections.reverseOrder());
    // z, a, C
   
    // Case-insensitive reverse-order sort
    Collections.sort(list, String.CASE_INSENSITIVE_ORDER);
    Collections.reverse(list);
    // z, C, a

e351. 对列表进行操作

See also e349 建立一个列表.

// Create the lists
    List list1 = new ArrayList();
    List list2 = new ArrayList();
    
    // Add elements to the lists ...
    
    // Copy all the elements from list2 to list1 (list1 += list2)
    // list1 becomes the union of list1 and list2
    list1.addAll(list2);
    
    // Remove all the elements in list1 from list2 (list1 -= list2)
    // list1 becomes the asymmetric difference of list1 and list2
    list1.removeAll(list2);
    
    // Get the intersection of list1 and list2
    // list1 becomes the intersection of list1 and list2
    list1.retainAll(list2);
    
    // Remove all elements from a list
    list1.clear();
    
    // Truncate the list
    int newSize = 2;
    list1.subList(newSize, list1.size()).clear();

e1075. 创建一个确切型别的列表

Generics can be used to create a list that will hold only objects of a certain type. This example creates a list of Integer objects. 
List<Integer> intlist = new ArrayList<Integer>();
    
    intlist.add(new Integer(123));
    // intlist.add("hello");       <- Syntax error
A list declared to hold objects of a type T can also hold objects that extend from T. In this example, a list is created to hold Number objects. Both Integer and Float are subclasses of Number.
List<Number> numlist = new ArrayList<Number>();
    
    numlist.add(new Integer(123));
    numlist.add(new Float(123));
Note that although null is not a subclass of any type, if the collection supports null values, it can be added to the type-specific collection.
intlist.add(null);
A value retrieved from a type-specific list does not need to be casted. In this example, a URL value is retrieved and used without an explicit cast.
List<URL> urlList = new ArrayList<URL>();
    try {
        urlList.add(new URL("http://javaref.cn"));
    } catch (MalformedURLException e) {
    }
    String s = urlList.get(0).getHost();

Sets
--------------------------------------------------------------------------------
e352. 建立一个集合

A set is a collection that holds unique values. Adding a value that's already in the set has no effect.

// Create the set
    Set set = new HashSet();
    
    // Add elements to the set
    set.add("a");
    set.add("b");
    set.add("c");
    
    // Remove elements from the set
    set.remove("c");
    
    // Get number of elements in set
    int size = set.size();          // 2
    
    // Adding an element that already exists in the set has no effect
    set.add("a");
    size = set.size();              // 2
    
    // Determining if an element is in the set
    boolean b = set.contains("a");  // true
    b = set.contains("c");          // false
    
    // Iterating over the elements in the set
    Iterator it = set.iterator();
    while (it.hasNext()) {
        // Get element
        Object element = it.next();
    }
    
    // Create an array containing the elements in the set (in this case a String array)
    String[] array = (String[])set.toArray(new String[set.size()]);

e353. 操作一个集合

See also e352 建立一个集合. 
// Create the sets
    Set set1 = new HashSet();
    Set set2 = new HashSet();
    
    // Add elements to the sets ...
    
    // Copy all the elements from set2 to set1 (set1 += set2)
    // set1 becomes the union of set1 and set2
    set1.addAll(set2);
    
    // Remove all the elements in set1 from set2 (set1 -= set2)
    // set1 becomes the asymmetric difference of set1 and set2
    set1.removeAll(set2);
    
    // Get the intersection of set1 and set2
    // set1 becomes the intersection of set1 and set2
    set1.retainAll(set2);
    
    // Remove all elements from a set
    set1.clear();
 


e354. 建立一个保持插入次序的集合

Set set = new LinkedHashSet();
    
    // Add some elements
    set.add("1");
    set.add("2");
    set.add("3");
    set.add("2");
    
    // List the elements
    for (Iterator it=set.iterator(); it.hasNext(); ) {
        Object o = it.next();
    }
    // [1, 2, 3]

Hash Tables
--------------------------------------------------------------------------------
e355. 建立一个散列表

A hash table, or map, holds key/value pairs.

// Create a hash table
    Map map = new HashMap();    // hash table
    map = new TreeMap();        // sorted map
    
    // Add key/value pairs to the map
    map.put("a", new Integer(1));
    map.put("b", new Integer(2));
    map.put("c", new Integer(3));
    
    // Get number of entries in map
    int size = map.size();        // 2
    
    // Adding an entry whose key exists in the map causes
    // the new value to replace the old value
    Object oldValue = map.put("a", new Integer(9));  // 1
    
    // Remove an entry from the map and return the value of the removed entry
    oldValue = map.remove("c");  // 3
    
    // Iterate over the keys in the map
    Iterator it = map.keySet().iterator();
    while (it.hasNext()) {
        // Get key
        Object key = it.next();
    }
    
    // Iterate over the values in the map
    it = map.values().iterator();
    while (it.hasNext()) {
        // Get value
        Object value = it.next();
    }
e356. 建立一个维护插入次序的MAP 

Map map = new LinkedHashMap();
    
    // Add some elements
    map.put("1", "value1");
    map.put("2", "value2");
    map.put("3", "value3");
    map.put("2", "value4");
    
    // List the entries
    for (Iterator it=map.keySet().iterator(); it.hasNext(); ) {
        Object key = it.next();
        Object value = map.get(key);
    }
    // [1=value1, 2=value4, 3=value3]

e357. 自动从三列表中删除没有引用的元素

When a key is added to a map, the map will prevent the key from being garbage-collected. However, a weak map will automatically remove a key if the key is not being referenced by any other object. An example where this type of map might be useful is a registry where a registrant is automatically removed after it is garbage-collected. 
// Create the weak map
    Map weakMap = new WeakHashMap();
    
    // Add a key to the weak map
    weakMap.put(keyObject, valueObject);
    
    // Get all keys that are still being referenced
    Iterator it = weakMap.keySet().iterator();
    while (it.hasNext()) {
        // Get key
        Object key = it.next();
    }
The weak map does not automatically release the value if it is no longer used. To enable automatically release of the value, the value must be wrapped in a WeakReference object:
WeakReference weakValue = new WeakReference(valueObject);
    weakMap.put(keyObject, weakValue);
    
    // Get all keys that are still being referenced and check whether
    // or not the value has been garbage-collected
    it = weakMap.keySet().iterator();
    while (it.hasNext()) {
        // Get key
        Object key = it.next();
    
        weakValue = (WeakReference)weakMap.get(key);
        if (weakValue == null) {
            // Value has been garbage-collected
        } else {
            // Get value
            valueObject = weakValue.get();
        }
    }
 


e1076. 创建一个确切型别的Map 

Generics can be used to create a map that will hold only objects of a certain type. This example creates a map whose keys are Integer objects and values are String objects. 
Map<Integer, String> map = new HashMap<Integer, String>();
    
    map.put(1, "first");
    map.put(2, "second");
    // map.put(1, 2);       <- Syntax error
A map declared to hold objects of a type T can also hold objects that extend from T. In this example, a map is created to hold Number objects as keys. Both Integer and Float are subclasses of Number.
Map<Number, String> numMap = new HashMap<Number, String>();
    
    numMap.put(.5, "half");
    numMap.put(1, "first");
Note that although null is not a subclass of any type, if the collection supports null values, it can be added to the type-specific collection.
map.put(null, null);
A value retrieved from a type-specific collection does not need to be casted. In this example, a URL value is retrieved and used without an explicit cast.
Map<String, URL> urlMap = new HashMap<String, URL>();
    try {
        urlMap.put("java", new URL("http://javaref.cn"));
    } catch (MalformedURLException e) {
    }
    String s = urlMap.get("java").getHost();

Sorted Collections
--------------------------------------------------------------------------------
e358. 建立一个排序的集合

A sorted set is a set that maintains its items in a sorted order. Inserts and retrievals are more expensive in a sorted set but iterations over the set is always in order.

See also e352 建立一个集合.

// Create the sorted set
    SortedSet set = new TreeSet();
    
    // Add elements to the set
    set.add("b");
    set.add("c");
    set.add("a");
    
    // Iterating over the elements in the set
    Iterator it = set.iterator();
    while (it.hasNext()) {
        // Get element
        Object element = it.next();
    }
    // The elements are iterated in order: a, b, c
    
    // Create an array containing the elements in a set (in this case a String array).
    // The elements in the array are in order.
    String[] array = (String[])set.toArray(new String[set.size()]);

e359. 对数组进行排序

int[] intArray = new int[] {4, 1, 3, -23};
    Arrays.sort(intArray);
    // [-23, 1, 3, 4]
   
    String[] strArray = new String[] {"z", "a", "C"};
    Arrays.sort(strArray);
    // [C, a, z]
   
    // Case-insensitive sort
    Arrays.sort(strArray, String.CASE_INSENSITIVE_ORDER);
    // [a, C, z]
   
    // Reverse-order sort
    Arrays.sort(strArray, Collections.reverseOrder());
    // [z, a, C]
   
    // Case-insensitive reverse-order sort
    Arrays.sort(strArray, String.CASE_INSENSITIVE_ORDER);
    Collections.reverse(Arrays.asList(strArray));
    // [z, C, a]

e360. 在一个排序数组中查找一个元素

// Create an array with an ordered list of strings
    String[] sortedArray = new String[]{"ant", "bat", "cat", "dog"};
   
    // Search for the word "cat"
    int index = Arrays.binarySearch(sortedArray, "cat");    // 2
   
    // Search for a non-existent element
    index = Arrays.binarySearch(sortedArray, "cow");        // -4
This example also works if the element is a primitive type.

// Create an array with an ordered list of numbers
    int[] sortedIntArray = new int[]{1, 2, 3, 5, 7};
    
    // Search for 6
    index = Arrays.binarySearch(sortedIntArray, 6);         // -5

A negative return value indicates that the element is not in the list. However, the actual return value can be used to determine where that non-existent element should be inserted in the list if that were desired; see e361 在一个排序数组中插入元素.

e1077. 在一个排序数组中插入元素

This example demonstrates how to determine the index at which an element should be inserted into a sorted array. Although binarySearch() is used to locate existent elements, it can also be used to determine the insert index for non-existent elements. Specifically, the insertion index is computed in the following way: insert-index = (-return-value)-1

// Create anarray with an ordered list of items
    String[] sortedArray = new String[]{"ant", "bat", "cat", "dog"};
    
    // Search for a non-existent item and then insert it
    int index = Arrays.binarySearch(sortedArray, "cow");
    if (index < 0) {
        // Compute the insert index
        int insertIndex = -index-1;
    
        // Insert the new item into sortedArray. The example here creates
        // a new larger array to hold the new item.
        String[] newSortedArray = new String[sortedArray.length+1];
        System.arraycopy(sortedArray, 0, newSortedArray, 0, insertIndex);
        System.arraycopy(sortedArray, insertIndex,
                         newSortedArray, insertIndex+1,
                         sortedArray.length-insertIndex);
        newSortedArray[insertIndex] = "cow";
        sortedArray = newSortedArray;
    }

e361. 在一个排序列表中查找一个元素

// Create a list with an ordered list of strings
    List sortedList = new LinkedList();
    sortedList.addAll(Arrays.asList(new String[]{"ant", "bat", "cat", "dog"}));
   
    // Search for the word "cat"
    int index = Collections.binarySearch(sortedList, "cat");    // 2
   
    // Search for a non-existent element
    index = Collections.binarySearch(sortedList, "cow");        // -4
A negative return value indicates that the element is not in the list. However, the actual return value can be used to determine where that non-existent element should be inserted in the list if that were desired; see e363 在一个排序列表中插入元素.

e362. 在一个排序列表中插入元素

This example demonstrates how to determine the index at which an element should be inserted into a sorted list. Although binarySearch() is used to locate existent elements, it can also be used to determine the insert index for non-existent elements. Specifically, the insertion index is computed in the following way: insert-index = (-return-value)-1

// Create a list with an ordered list of items
    List sortedList = new LinkedList();
    sortedList.addAll(Arrays.asList(new String[]{"ant", "bat", "cat", "dog"}));
    
    // Search for the non-existent item
    int index = Collections.binarySearch(sortedList, "cow");      // -4
    
    // Add the non-existent item to the list
    if (index < 0) {
        sortedList.add(-index-1, "cow");
    }

Bits
--------------------------------------------------------------------------------
e363. 对一个位指针进行位操作

The BitSet class implements a bit-vector of an arbitrary size. It automatically grows dynamically. This example demonstrates how to create and use a BitSet.

The BigInteger class also support bitwise operations (see e129 对Big Integer进行位操作). However, a BigInteger object is immutable where a BitSet is mutable.

// Create the bitset
    BitSet bits = new BitSet();
    
    // Set a bit on
    bits.set(2);                        // 100 = decimal 4
    
    // Retrieving the value of a bit
    boolean b = bits.get(0);            // false
    b = bits.get(2);                    // true
    
    // Clear a bit
    bits.clear(1);
    
    // Setting a range of bits
    BitSet bits2 = new BitSet();
    bits2.set(1, 4);                    // 1110
    
    // And'ing two bitsets
    bits.and(bits2);                    // 0100
    
    // Xor'ing two bitsets
    bits.xor(bits2);                    // 1010
    
    // Flip all bits in the bitset
    bits.flip(0, bits.length());        // 0101
    
    // Andnot'ing two bitsets
    bits.andNot(bits2);                 // 0001
    
    // Or'ing two bitsets
    bits.or(bits2);                     // 1111

e364. 在BitSet和字节数组之间进行转换

There are no default methods for converting a BitSet to and from a byte array. This example implements two methods to do the conversion. These methods make it possible to easily work with both BitSet and BigInteger and take advantage of their capabilities when needed.

// Returns a bitset containing the values in bytes.
    // The byte-ordering of bytes must be big-endian which means the most significant bit is in element 0.
    public static BitSet fromByteArray(byte[] bytes) {
        BitSet bits = new BitSet();
        for (int i=0; i<bytes.length*8; i++) {
            if ((bytes[bytes.length-i/8-1]&(1<<(i%8))) > 0) {
                bits.set(i);
            }
        }
        return bits;
    }
    
    // Returns a byte array of at least length 1.
    // The most significant bit in the result is guaranteed not to be a 1
    // (since BitSet does not support sign extension).
    // The byte-ordering of the result is big-endian which means the most significant bit is in element 0.
    // The bit at index 0 of the bit set is assumed to be the least significant bit.
    public static byte[] toByteArray(BitSet bits) {
        byte[] bytes = new byte[bits.length()/8+1];
        for (int i=0; i<bits.length(); i++) {
            if (bits.get(i)) {
                bytes[bytes.length-i/8-1] |= 1<<(i%8);
            }
        }
        return bytes;
    }

Property Files
--------------------------------------------------------------------------------
e365. 读取和写入一个属性文件

// Read properties file.
    Properties properties = new Properties();
    try {
        properties.load(new FileInputStream("filename.properties"));
    } catch (IOException e) {
    }
   
    // Write properties file.
    try {
        properties.store(new FileOutputStream("filename.properties"), null);
    } catch (IOException e) {
    }
Here is an example of the contents of a properties file:

# a comment
    ! a comment
    
    a = a string
    b = a string with escape sequences \t \n \r \\ \" \' \ (space) \u0123
    c = a string with a continuation line \
        continuation line
    d.e.f = another string

e366. 获取并设置属性

 String string = properties.getProperty("a.b");
    properties.setProperty("a.b", "new value");

Timers
--------------------------------------------------------------------------------
e367. 调度一个计时器任务，使其在特定时间执行

int numberOfMillisecondsInTheFuture = 10000; // 10 sec
    Date timeToRun = new Date(System.currentTimeMillis()+numberOfMillisecondsInTheFuture);
    Timer timer = new Timer();
   
    timer.schedule(new TimerTask() {
            public void run() {
                // Task here ...
            }
        }, timeToRun);

e368. 调度一个计时器任务，让它重复执行

 int delay = 5000;   // delay for 5 sec.
    int period = 1000;  // repeat every sec.
    Timer timer = new Timer();
   
    timer.scheduleAtFixedRate(new TimerTask() {
            public void run() {
                // Task here ...
            }
        }, delay, period);

Time
--------------------------------------------------------------------------------
e369. 获取当前时间

Calendar cal = new GregorianCalendar();
   
    // Get the components of the time
    int hour12 = cal.get(Calendar.HOUR);            // 0..11
    int hour24 = cal.get(Calendar.HOUR_OF_DAY);     // 0..23
    int min = cal.get(Calendar.MINUTE);             // 0..59
    int sec = cal.get(Calendar.SECOND);             // 0..59
    int ms = cal.get(Calendar.MILLISECOND);         // 0..999
    int ampm = cal.get(Calendar.AM_PM);             // 0=AM, 1=PM

e370. 获取另一个时区的当前时间

// Get the current time in Hong Kong
    Calendar cal = new GregorianCalendar(TimeZone.getTimeZone("Hongkong"));
   
    int hour12 = cal.get(Calendar.HOUR);         // 0..11
    int minutes = cal.get(Calendar.MINUTE);      // 0..59
    int seconds = cal.get(Calendar.SECOND);      // 0..59
    boolean am = cal.get(Calendar.AM_PM) == Calendar.AM;
   
    // Get the current hour-of-day at GMT
    cal.setTimeZone(TimeZone.getTimeZone("GMT"));
    int hour24 = cal.get(Calendar.HOUR_OF_DAY);  // 0..23
   
    // Get the current local hour-of-day
    cal.setTimeZone(TimeZone.getDefault());
    hour24 = cal.get(Calendar.HOUR_OF_DAY);      // 0..23

e371. 检索所有可用时区的信息

This example lists all time zones known by the JDK.

Date today = new Date();
    
    // Get all time zone ids
    String[] zoneIds = TimeZone.getAvailableIDs();
    
    // View every time zone
    for (int i=0; i<zoneIds.length; i++) {
        // Get time zone by time zone id
        TimeZone tz = TimeZone.getTimeZone(zoneIds[i]);
    
        // Get the display name
        String shortName = tz.getDisplayName(tz.inDaylightTime(today), TimeZone.SHORT);
        String longName = tz.getDisplayName(tz.inDaylightTime(today), TimeZone.LONG);
    
        // Get the number of hours from GMT
        int rawOffset = tz.getRawOffset();
        int hour = rawOffset / (60*60*1000);
        int min = Math.abs(rawOffset / (60*1000)) % 60;
    
        // Does the time zone have a daylight savings time period?
        boolean hasDST = tz.useDaylightTime();
    
        // Is the time zone currently in a daylight savings time?
        boolean inDST = tz.inDaylightTime(today);
    }

Here's a few time zone entries:

Id, Short Name, Long Name, Hour:Time from GMT
    
    ACT, CST, Central Standard Time (Northern Territory) 9:30
    AET, EST, Eastern Summer Time (New South Wales) 10:0
    AGT, ART, Argentine Time -3:0
    ART, EET, Eastern European Time 2:0
    AST, AKST, Alaska Standard Time -9:0
    Africa/Abidjan, GMT, Greenwich Mean Time 0:0
    Africa/Accra, GMT, Greenwich Mean Time 0:0
    Africa/Addis_Ababa, EAT, Eastern African Time 3:0
    Africa/Algiers, CET, Central European Time 1:0
    Africa/Asmera, EAT, Eastern African Time 3:0
    Africa/Bamako, GMT, Greenwich Mean Time 0:0
    Africa/Bangui, WAT, Western African Time 1:0

e372. 在时区之间进行时间转换

 There is a convenient setTimeZone() method in the Calendar object. However, it doesn't always return the correct results when used after a calendar field is set. This example demonstrates a more reliable way to convert a specific time from one time zone to another. It involves creating two Calendar instances and transfering the UTC (Coordinate Universal Time) from one to the other. The UTC is a representation of time and date that is independent of time zones.

// Given a local time of 10am, get the time in Japan
    // Create a Calendar object with the local time zone
    Calendar local = new GregorianCalendar();
    local.set(Calendar.HOUR_OF_DAY, 10);               // 0..23
    local.set(Calendar.MINUTE, 0);
    local.set(Calendar.SECOND, 0);
    
    // Create an instance using Japan's time zone and set it with the local UTC
    Calendar japanCal = new GregorianCalendar(TimeZone.getTimeZone("Japan"));
    japanCal.setTimeInMillis(local.getTimeInMillis());
    
    // Get the foreign time
    int hour = japanCal.get(Calendar.HOUR);            // 3
    int minutes = japanCal.get(Calendar.MINUTE);       // 0
    int seconds = japanCal.get(Calendar.SECOND);       // 0
    boolean am = japanCal.get(Calendar.AM_PM) == Calendar.AM; //true
    
    
    // Given a time of 10am in Japan, get the local time
    japanCal = new GregorianCalendar(TimeZone.getTimeZone("Japan"));
    japanCal.set(Calendar.HOUR_OF_DAY, 10);            // 0..23
    japanCal.set(Calendar.MINUTE, 0);
    japanCal.set(Calendar.SECOND, 0);
    
    // Create a Calendar object with the local time zone and set
    // the UTC from japanCal
    local = new GregorianCalendar();
    local.setTimeInMillis(japanCal.getTimeInMillis());
    
    // Get the time in the local time zone
    hour = local.get(Calendar.HOUR);                   // 5
    minutes = local.get(Calendar.MINUTE);              // 0
    seconds = local.get(Calendar.SECOND);              // 0
    am = local.get(Calendar.AM_PM) == Calendar.AM;     // false

Dates
--------------------------------------------------------------------------------
e373. 获取当前日期

Calendar cal = new GregorianCalendar();
   
    // Get the components of the date
    int era = cal.get(Calendar.ERA);               // 0=BC, 1=AD
    int year = cal.get(Calendar.YEAR);             // 2002
    int month = cal.get(Calendar.MONTH);           // 0=Jan, 1=Feb, ...
    int day = cal.get(Calendar.DAY_OF_MONTH);      // 1...
    int dayOfWeek = cal.get(Calendar.DAY_OF_WEEK); // 1=Sunday, 2=Monday, ...

e374. 为特定日期建立一个Date对象

 Calendar xmas = new GregorianCalendar(1998, Calendar.DECEMBER, 25);
    Date date = xmas.getTime();

e375. 确定一个月当中的天数

This example uses the Calendar class to determine the number of days in the month of a particular year.

// Create a calendar object of the desired month
    Calendar cal = new GregorianCalendar(1999, Calendar.FEBRUARY, 1);
    
    // Get the number of days in that month
    int days = cal.getActualMaximum(Calendar.DAY_OF_MONTH); // 28