String类型的toString（）返回的是什么

String s1 = " hello";
String s2 = s1.substring(2,3);
String s3 = s1.toString();
String s4 = new StringBuffer(s1).toString();
String s5 = " hello";
System.out.println(s1 == s3);
System.out.println(s1 == s4);
System.out.println(s2 == s4);
System.out.println(s1 == s2);
System.out.println(s1 == s5);

public String toString() {
return this;
}