String assignments and output

Question

Given the following code:

String str1 = new String("Hello");
String str2 = str1;
String str3 = new String(str1);
String str4 = str3;
str4 += " World ";
if (str3==str4)
     System.out.println(“one”);
if (str3.equals(str4))
     System.out.println(“two”);
if (str1==str2)
     System.out.println(“three”); 
if (str3.equals(str2))
     System.out.println(“four”);

The output is : Three and Four

I don't get it.. we just did str3 == str4 . how can they NOT refer to the same object..? str3 == str4 seem to be false and I dont get why. In addition, the str3.equals(str4) also return false but I guess that has something to do with the first thing I dont get.

Would love to get an explanation.

this is a duplicate, please browse the site for similar questions . :) — PermGenError, May 14 '13 at 14:14
this one..? http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java? That doesnt answer my question. — Rouki, May 14 '13 at 14:15
`str4 += " World ";` changes the value of `str4` - it's as simple as that. After that statement, `str4` refers to a string with contents `Hello World ` but `str3` still refers to a string with contents `Hello`. So they're not only distinct string objects, but objects with different contents. — Jon Skeet, May 14 '13 at 14:17
This is not a duplicate. The OP clearly knows how to compare strings, but the confusion comes from misunderstanding of how object references work in conjunction with string immutability. — Sergey Kalinichenko, May 14 '13 at 14:17
This question is not about comparison but about the `+=` operator. — Moritz Petersen, May 14 '13 at 14:18
Because a `String` is immutable, the `+=` operator creates a new instance and assignes it to `str4`. Therefore `str4` is not equal `str3`. — Moritz Petersen, May 14 '13 at 14:18

score 7 · Accepted Answer · answered May 14 '13 at 14:20

7

Because a String is immutable, the += operator creates a new instance and assignes it to str4. Therefore str4 is not equal str3.

answered May 14 '13 at 14:20

Moritz Petersen

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score 3 · Answer 2 · answered May 14 '13 at 14:21

Here is what happens: str3 and str4 start off referencing the same object. However, since Java String is immutable and thus cannot be modified in-place, this line

str4 += " World ";

results in assigning str4 a reference to a brand-new object. That is why the subsequent comparison str3==str4 fails.

String assignments and output

2 Answers2