==和equals的对比 [面试题]

1.既可判断基本类型，也可以判断引用类型2.基本：判断值是否相等

int a = 10;
        double d = 10.0;
        boolean c;
        c = a==d;
        System.out.println(c);    //true

3.引用：判断地址是否相等

class B {}
class A extends B {}
A a = new A();
        A b = a;
        A c = b;
        System.out.println(a==c);//true
        System.out.println(b==c);//true
        B obj = a;
        System.out.println(obj==c);//true

默认判断地址是否相等，子类中往往重写该方法，用于判断内容是否相等。例如Integer、String

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

//String类重写equals
public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }
//比较俩个字符串值是否相同
"hello".equals("abc");//false
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		Integer integer1 = new Integer(1000);
        Integer integer2 = new Integer(1000);
        System.out.println(integer1==integer2);//false
        System.out.println(integer1.equals(integer2));//true

        String s1 = new String("ssy");
        String s2 = new String("ssy");
        System.out.println(s1==s2);//false
        System.out.println(s1.equals(s2));//true