字符串相加不一定相等

public class demo01 {
      public static void main(String[] args) {
            String s1 = "hellojava";
	    String s2 = "hello";
	    String s3 = "java";
	    String s4 = "hello" + s3;
	    String s5 = s2 + "java";
	    String s6 = s2 + s3;
	    String s7 = "hello" + "java";
	    String s8 = new String("helloworld");
	    System.out.println(s4);
	    System.out.println(s1 == s4);
	    System.out.println(s5);
	    System.out.println(s1 == s5);
	    System.out.println(s6);
	    System.out.println(s1 == s6);
	    System.out.println(s7);
	    System.out.println(s1 == s7);
	    System.out.println(s8);
	    System.out.println(s1 == s8);
	 }
}
public class demo01 {
      public static void m