== difference in java with the equal

String str1 = new String("str");
        String str2 = new String("str");
        System.out.println ( "== Comparison:" + (str1 == str2));
        System.out.println("equal比较："+ str1.equals(str2));
        String str3 = "str1";
        String str4 = "str1";
        System.out.println ( "== Comparison:" + (Str3 == str4));
        System.out.println("equal比较："+ str3.equals(str4));

Output:
== Comparison: to false
equal comparison: true
== Comparison: to true
comparison equal: true

The results can be found, equals to instantiate objects by a new, or by automatic packing to instantiate objects, the result is true, and returns results when used autoboxing == true, the use of new time returns false. It can be found through the source code:

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

Here the definition of equals and == are equivalent, but the results Why is not the same cause?
The reason is that the String class equals rewritten to :

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;
    }

Here again need attention equals five points:

1   自反性：对任意引用值X，x.equals(x)的返回值一定为true.
2   对称性：对于任何引用值x,y,当且仅当y.equals(x)返回值为true时，x.equals(y)的返回值一定为true;
3   传递性：如果x.equals(y)=true, y.equals(z)=true,则x.equals(z)=true
4   一致性：如果参与比较的对象没任何改变，则对象比较的结果也不应该有任何改变
5   非空性：任何非空的引用值X，x.equals(null)的返回值一定为false

最后总结得出结论：

equal:是用来比较两个对象内部的内容是否相等的，由于所有的类都是继承自java.lang.Object类的，所以如果没有对该方法进行覆盖的话，调用
的仍然是Object类中的方法，而Object中的equal方法返回的却是==的判断，因此，如果在没有进行该方法的覆盖后，调用该方法是没有
任何意义的。在java面向对象的处理中我们一般在javabean中都要选择重写equals方法，使用hibernate后，我们要生成数据库的映射文件与实体类，这是我们就最好在实体类中进行equals方法的重写，重写时我们可以根据自己的定义来实现该方法只要遵守那五条原则，例如对于一个student类我们定义只要在学号相同时我们就认为这两个对象时相等的；同时我们还要重写hashcode方法。

==: is used to determine two object addresses are the same, i.e., whether or not refer to the same object . Compare a pointer operation in the true sense.

== difference in java with the equal

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