public class TestString {

    void test1() {

        // TODO Auto-generated method stub

        String str = new String();

        str = "12345";

        String STR = "12345";

        System.out.println(str == STR);

        String str_1 = str;

        String str_2 = null;

        str_2 = str;

        String str_3 = new String();

        str_3 = str;

        String str_4 = new String(str);

        System.out.println("str:" + str);

        System.out.println("str_1:" + str_1);

        System.out.println("str == str_1:" + str == str_1);

        System.out.println("str_2:" + str_2);

        System.out.println("str == str_2:" + str == str_2);

        System.out.println("str_3:" + str_3);

        System.out.println("str == str_3:" + str == str_3);

        System.out.println("str_4:" + str_4);

        System.out.println("str == str_4:" + str == str_4);

        str = "2345678";

        System.out.println("str:" + str);

        System.out.println("str_1:" + str_1);

        System.out.println("str == str_1:" + str == str_1);

        System.out.println("str_2:" + str_2);

        System.out.println("str == str_2:" + str == str_2);

        System.out.println("str_3:" + str_3);

        System.out.println("str == str_3:" + str == str_3);

        System.out.println("str_4:" + str_4);

        System.out.println("str == str_4:" + str == str_4);

    }

    static void test2(){//false

        String str = new String("123");

        String str_2 = new String("123");

        System.out.println( str == str_2);

    }

    static void test3(){//false

        String str =  "123";

        String str_2 = new String("123");

        System.out.println( str == str_2);

    }

    static void test4(){//true

        String str =  "123";

        String str_2 = "123";

        System.out.println( str == str_2);

    }    

    public static void main(String[] args) {

        test2();

        test3();

        test4();

    }

}