内部类

// innerclasses/Sequence.java
// Holds a sequence of Objects
interface Selector {
    boolean end();
    Object current();
    void next();
}
public class Sequence {
    private Object[] items;
    private int next = 0;
    public Sequence(int size) {
        items = new Object[size];
    }
    public void add(Object x) {
        if(next < items.length)
            items[next++] = x;
    }
    private class SequenceSelector implements Selector {
        private int i = 0;
        @Override
        public boolean end() { return i == items.length; }
        @Override
        public Object current() { return items[i]; }
        @Override
        public void next() { if(i < items.length) i++; }
    }
    public Selector selector() {
        return new SequenceSelector();
    }
    public static void main(String[] args) {
        Sequence sequence = new Sequence(10);
        for(int i = 0; i < 10; i++)
            sequence.add(Integer.toString(i));
        Selector selector = sequence.selector();
        while(!selector.end()) {
            System.out.print(selector.current() + " ");
            selector.next();
        }
    }
}

// innerclasses/DotThis.java
// Accessing the outer-class object
public class DotThis {
    void f() { System.out.println("DotThis.f()"); }

    public class Inner {
        public DotThis outer() {
            return DotThis.this;
            // A plain "this" would be Inner‘s "this"
        }
    }

    public Inner inner() { return new Inner(); }

    public static void main(String[] args) {
        DotThis dt = new DotThis();
        DotThis.Inner dti = dt.inner();
        dti.outer().f();
    }
}

// innerclasses/Parcel3.java
// Using .new to create instances of inner classes
public class Parcel3 {
    class Contents {
        private int i = 11;
        public int value() { return i; }
    }
    class Destination {
        private String label;
        Destination(String whereTo) { label = whereTo; }
        String readLabel() { return label; }
    }
    public static void main(String[] args) {
        Parcel3 p = new Parcel3();
        // Must use instance of outer class
        // to create an instance of the inner class:
        Parcel3.Contents c = p.new Contents();
        Parcel3.Destination d =
                p.new Destination("Tasmania");
    }
}

// innerclasses/TestParcel.java
class Parcel4 {
    private class PContents implements Contents {
        private int i = 11;
        @Override
        public int value() { return i; }
    }
    protected final class PDestination implements Destination {
        private String label;
        private PDestination(String whereTo) {
            label = whereTo;
        }
        @Override
        public String readLabel() { return label; }
    }
    public Destination destination(String s) {
        return new PDestination(s);
    }
    public Contents contents() {
        return new PContents();
    }
}
public class TestParcel {
    public static void main(String[] args) {
        Parcel4 p = new Parcel4();
        Contents c = p.contents();
        Destination d = p.destination("Tasmania");
        // Illegal -- can‘t access private class:
        //- Parcel4.PContents pc = p.new PContents();
    }
}