java中equals与==的对比

class B {
    private int num;
    B() {
        num = 1;
    }
}
B a = new B();
B b = new B();
System.out.println(a.equals(b));//false

String str_a = new String("abc");
String str_b = new String("abc");
System.out.println(str_a.equals(str_b)); //true
System.out.println(str_a==str_b); // false

StringBuffer sb1 = new StringBuffer("a");
StringBuffer sb2 = new StringBuffer("a");
System.out.println(sb1.equals(sb2)); //false
System.out.println(sb1==sb2); // false

class A {
    private int a;
    A() {
        a = 1;
    }
    public boolean equals(A b) {
        if (a == b.a)
            return true;
        else
            return false;
    }
}
A a = new A();
A b = new A();
System.out.println(a.equals(b)); //true
System.out.println(a == b);    //false