equals和==

	public static void main(String[] args) {
	
		String s1 = new String("a");
		String s2 = new String("a");
		
		
		System.out.println(s1==s2);
		System.out.println(s1.equals(s2));
		
		s1=s2;
		
		System.out.println(s1==s2);
		System.out.println(s1.equals(s2));
		
	}

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结果
false
true
true
true
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说明: ==比较是两个对象的引用，equals是比较对象的内容。
Object中equals 源码

 public boolean equals(Object obj) {
	return (this == obj);
    }

如果对象没有重写equals方法，则==和equals是相同的。

String 重写equals方法

  public boolean equals(Object anObject) {
	if (this == anObject) {
	    return true;
	}
	if (anObject instanceof String) {
	    String anotherString = (String)anObject;
	    int n = count;
	    if (n == anotherString.count) {
		char v1[] = value;
		char v2[] = anotherString.value;
		int i = offset;
		int j = anotherString.offset;
		while (n-- != 0) {
		    if (v1[i++] != v2[j++])
			return false;
		}
		return true;
	    }
	}
	return false;
    }