Java：字符串引用得到意外的输出

Question

My code is :

String str = "Hello";
String str1;
str1 = str;
str1 = str1 + " World";
System.out.println(str1);
System.out.println(str);

The output I get is :

Hello World
Hello

The output I was expecting is Hello World for both the cases because according to my understanding after str1 = str both objects are referencing to same location so if I change the content of one object other should also get affected.

So, is str1 = str1 + " World"; creating a new string object at different memory loction.?

Answer 1

Strings are immutable. When you do str1 = str1 + " World"; , you're creating a totally new String and updating str1 's reference to it. You're never re-asigning str so this IS the expected behaviour.

Answer 2

The point is that the + operator creates a new String object, effectively concatenating the operands. It does not modify the object str .

You might want to read http://javarevisited.blogspot.co.at/2010/10/why-string-is-immutable-in-java.html , which explains the concept of immutable strings quite nicely.

The class StringBuffer might also be worth noting, which is optimized for long, cascaded concatenations.

Answer 3

In Java String is immutable.

str1 = str1 + " World"; It will create a new instance and assign to str1.

Answer 4

java.lang.String is immutable

str1 = str1 + " World";

This code will make str1 reference to a new created String object "Hello World" , and str still reference to the "Hello" object.

Answer 5

引用原始日期类型总是创建一个新的对象

Answer 6

No, this is the right out put as in the code you had init the string

String str = "Hello";
 String str1;

Then you say

str1 = str;

it means str1 contains "Hello"

str1 = str1 + " World";

The above line will print "Hello World" because str1 having "Hello" and " World" is given there as a new string

and when you print str it is containing the word "Hello"

System.out.println(str);

Answer 7

What is so special about String in here, It's the common behavior for all. It's about the assignment operator (in line number 4 you assigning it again). Your code is similar to below.

Person p1 = new Person("Stack");
Person p2;
p2 = p1;
p1 = new Person("OverFlow");

What would this Print? Would both print "Overflow"?

From your words.

The output I was expecting is Hello World for both the cases because according to my understanding after str1 = str both objects are referencing to same location so if I change the content of one object other should also get affected.

Line 3: str1 = str;

As per Line number 3 you are correct.

Line 4: str1 = str1 + " World";

But in line number 4 you are not changing the content, you are reassigning str1 to a different object.

Here str, str1 and p1 are Object references. Not real objects. You would get same content if you are changing the object. Like

p1 = p1.setName("Overflow");