JDK中各类类的例子

内部类的例子

AbstractList类有一个Itr内部类。它是Iterator接口的实现，可以逐个获取集合的元素：

private class Itr implements Iterator<E> {
	int cursor = 0;
	int lastRet = -1;
	int expectedModCount = modCount;

	public boolean hasNext() {
    		return cursor != size();
	}

	public E next() {
    	checkForComodification();
    	try {
        	int i = cursor;
        	E next = get(i);
        	lastRet = i;
        	cursor = i + 1;
        	return next;
    	} catch (IndexOutOfBoundsException e) {
        	checkForComodification();
        	throw new NoSuchElementException(e);
    	}
	}

	public void remove() {
    	if (lastRet < 0)
        	throw new IllegalStateException();
    	checkForComodification();

    	try {
        	AbstractList.this.remove(lastRet);
        	if (lastRet < cursor)
            	cursor--;
        	lastRet = -1;
        	expectedModCount = modCount;
    	} catch (IndexOutOfBoundsException e) {
   	     throw new ConcurrentModificationException();
    	}
	}

	final void checkForComodification() {
    	if (modCount != expectedModCount)
        	throw new ConcurrentModificationException();
	}
}

它在迭代器方法中使用：

public Iterator<E> iterator() {
	return new Itr();
}

这就是AbstractList的任何后代如何获得现成的迭代器。而如果需要自定义迭代器，可以实现自己的继承Iterator或Itr的类，然后重写迭代器方法。例如，这就是ArrayList类所做的。

Itr类是非静态的。因此，Itr对象具有对AbstractList实例的引用，并且可以访问其方法（size、get、remove）。

静态嵌套类的示例

Integer类有一个IntegerCache嵌套类。

private static class IntegerCache {
	static final int low = -128;
	static final int high;
	static final Integer[] cache;
	static Integer[] archivedCache;

	static {
    	int h = 127;
    	String integerCacheHighPropValue =
        	VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
    	if (integerCacheHighPropValue != null) {
        	try {
            	h = Math.max(parseInt(integerCacheHighPropValue), 127);
            	h = Math.min(h, Integer.MAX_VALUE - (-low) -1);
        	} catch( NumberFormatException nfe) {
        	}
    	}
    	high = h;

    	VM.initializeFromArchive(IntegerCache.class);
    	int size = (high - low) + 1;

    	if (archivedCache == null || size > archivedCache.length) {
        	Integer[] c = new Integer[size];
        	int j = low;
        	for(int i = 0; i < c.length; i++) {
            	c[i] = new Integer(j++);
        	}
        	archivedCache = c;
    	}
    	cache = archivedCache;
    	assert IntegerCache.high >= 127;
}

	private IntegerCache() {}
}

IntegerCache封装了创建缓存和存储缓存范围以及缓存值本身的功能。因此，与缓存相关的所有内容都保存在一个单独的类中。这使得阅读和修改代码变得更加容易。使用该类的代码：

public static Integer valueOf(int i) {
	if (i >= IntegerCache.low && i <= IntegerCache.high)
    		return IntegerCache.cache[i + (-IntegerCache.low)];
	return new Integer(i);
}

IntegerCache类不访问Integer类的非静态字段和方法。此外，它只能在静态valueOf方法中访问。也就是说，它绑定到Integer类本身，而不是它的各个实例。这意味着IntegerCache是静态的。

匿名内部类的例子

作为匿名类的示例，让我们以InputStream及其静态nullInputStream方法为例：

public static InputStream nullInputStream() {
    return new InputStream() {
    	private volatile boolean closed;

    	private void ensureOpen() throws IOException {
        	if (closed) {
            		throw new IOException("Stream closed");
        	}
    	}

    	@Override
    	public int available () throws IOException {
        	ensureOpen();
        	return 0;
    	}

    	@Override
    	public int read() throws IOException {
        	ensureOpen();
        	return -1;
    	}

    	@Override
    	public int read(byte[] b, int off, int len) throws IOException {
        	Objects.checkFromIndexSize(off, len, b.length);
        	if (len == 0) {
            		return 0;
        	}
        	ensureOpen();
        	return -1;
    	}

    	@Override
    	public byte[] readAllBytes() throws IOException {
        	ensureOpen();
        	return new byte[0];
    	}

    	@Override
    	public int readNBytes(byte[] b, int off, int len)throws IOException {
        	Objects.checkFromIndexSize(off, len, b.length);
        	ensureOpen();
        	return 0;
    	}

    	@Override
   	 public byte[] readNBytes(int len) throws IOException {
        	if (len < 0) {
            		throw new IllegalArgumentException("len < 0");
        	}
        	ensureOpen();
        	return new byte[0];
    	}

    	@Override
    	public long skip(long n) throws IOException {
        	ensureOpen();
        	return 0L;
    	}

    	@Override
    	public void skipNBytes(long n) throws IOException {
        	ensureOpen();
        	if (n > 0) {
            		throw new EOFException();
        	}
    	}

    	@Override
    	public long transferTo(OutputStream out) throws IOException {
        	Objects.requireNonNull(out);
        	ensureOpen();
        	return 0L;
    	}

    	@Override
    	public void close() throws IOException {
        	closed = true;
    	}
    };
}

该方法返回一个空的InputStream，由匿名类实现。因为该类不应该有后代，所以我们将其设为匿名。

随着 Java Stream API 的加入，匿名类变得无处不在：所有的 lambda 表达式都是实现了一些函数式接口的匿名类。考虑一些例子。

AbstractStringBuilder类包含著名的StringBuilder和StringBuffer类的父类：

@Override
public IntStream chars() {
	return StreamSupport.intStream(
        	() -> {
            	byte[] val = this.value;
            	int count = this.count;
            	byte coder = this.coder;
            	return coder == LATIN1
                   	? new StringLatin1.CharsSpliterator(val, 0, count, 0)
                   	: new StringUTF16.CharsSpliterator(val, 0, count, 0);
        	},
        	Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED,
        	false);
}

Files类有一个用于将Closeable转换为Runnable 的方法：

private static Runnable asUncheckedRunnable(Closeable c) {
	return () -> {
    	try {
        	c.close();
    	} catch (IOException e) {
        	throw new UncheckedIOException(e);
    	}
	};
}

Class类有一个获取方法的字符串表示的方法：

private String methodToString(String name, Class<?>[] argTypes) {
	return getName() + '.' + name +
        	((argTypes == null || argTypes.length == 0) ?
        	"()" :
        	Arrays.stream(argTypes)
        	        .map(c -> c == null ? "null" : c.getName())
                	.collect(Collectors.joining(",", "(", ")")));
}