Set 的主要实现类：HashSet

public class TestSet {
	@Test
	public void TestHashSet() {
		Set set1 = new HashSet();
		set1.add(null);// 可以存null
		set1.add(1);
		set1.add(2);
		set1.add(new String("A"));
		set1.add(new String("A"));// 不可重复性（依赖于 equals()和 hashCode()方法）
		set1.add("B");
		System.out.println(set1);// [null, 1, A, 2, B]
		Set set2 = new HashSet();
		set2.add(2);
		set2.add(null);
		set2.add("B");
		set2.add(1);
		set2.add("A");
		System.out.println(set2);// [null, 1, A, 2, B],无序性
	}
}

public class TestSet2 {
	public static void main() {
		Set set = new HashSet();
		set.add(new Person1("aa"));
		set.add(new Person1("aa"));
		set.add(new Person2("aa"));
		set.add(new Person2("aa"));// Person2 没有重写 hashCode()方法，此时调用的是 Object类定义的hashCode()方法，算出哈希值不一样，所以可以存储
		System.out.println(set);// [Person2 [name=aa], Person2 [name=aa],Person1 [name=aa]]
	}
}
class Person1 {
	private String name;
	@Override
	public String toString() {
		return "Person1 [name=" + name + "]";
	}
	public Person1(String name) {
		super();
		this.name = name;
	}
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Person1 other = (Person1) obj;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}
}
class Person2 {
	private String name;
	public Person2(String name) {
		super();
		this.name = name;
	}
	@Override
	public String toString() {
		return "Person2 [name=" + name + "]";
	}
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Person2 other = (Person2) obj;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}
}