String objects passed by value and pass by reference

Recently looking at the stack and passed by value and passed by reference , these two concepts have a certain understanding , but there is confusion treat string object Shanghai , please help correct me. I think str1, after operate should abcdef, str2 after the method should operate as abc, now str1 through method , the result is still as abc. Can str1 is new out of the object, passing should be a reference , operation, should modify the value of the heap , and then let str1 points to its value , ask what is wrong with my understanding , please correct me criticism . Thank you.

  static void operate(String str) {
      str += "def";
  }
  	
  static void mapAdd(Map<String, String> map) {
      map.put("aa", "aa");
  }
  	
  public static void main(String[] args) {
      String str1 = new String("abc");
      String str2 = "abc";
  		
      operate(str1);
      // str1指向了堆中new出的对象abc，像operate传递时，应该传递的是str1的引用
      // 在方法中操作后，str1应该指向到操作后的新的对象，也就是abcdef
      System.out.println(str1);
  		
      operate(str2);
      // str2为栈中的对象，传递的就是abc数值，方法操作完后，新的值仅仅是方法中的局部变量
      // 值未带出到main方法中来，所以str2还应该是abc
      System.out.println(str2);
  		
      Map<String, String> map = new HashMap<String, String>();
      map.put("bb", "bb");
  		
      // 这里没有疑问，mapAdd中传递的map为对象，传递的是引用，操作后map指向了操作后堆中的map值
      // 操作后，main方法中map中有两队值。
      mapAdd(map);
      for (String str : map.keySet()) {
          System.out.println(str);
      }
  }
  

------ Solution ------------------------------------- after -------
str1 when the object reference is passed to a method that can change the properties of this object , and can return the appropriate change , but the object reference string point is never becomes .
As you wrote back that no new object variable to change its value can be written like this :
class a (int i)
{
i = i +1
}
void main:
int b = 3
a A = new a ();
A.add (b); sys (b)
------ Solution ------------------------- -------------------
okay with heap or pool
value can not be modified because the String object
str + = "def";
This sentence will be executed after a new object on the heap , the value is abcdef, and then update the reference str , while str1 not changed
------ For reference only -------- -------------------------------

above this sentence, can be explained by the map passed back you, please explain the different transmission and map passed str1 , thank you.
------ For reference only -------------------------------------- -

  
  	static void operate(StringBuffer buffer) {
  		System.out.println(buffer==buffer1);
  		buffer.append("def");
  		System.out.println(buffer==buffer1);
  	}
  	static void operate(String str) {
  		System.out.println(str==str2);
  		str+="def";
  		System.out.println(str==str2);
  	}
  	static void mapAdd(Map<String,String> map) {
  		map.put("aa","aa");
  	}
  	static StringBuffer buffer1=new StringBuffer("abc");
  	static String str2="abc";
  	public static void main(String[] args) {
  		operate(buffer1);
  		// str1指向了堆中new出的对象abc，像operate传递时，应该传递的是str1的引用
  		// 在方法中操作后，str1应该指向到操作后的新的对象，也就是abcdef
  		System.out.println(buffer1);
  		operate(str2);
  		// str2为栈中的对象，传递的就是abc数值，方法操作完后，新的值仅仅是方法中的局部变量
  		// 值未带出到main方法中来，所以str2还应该是abc
  		System.out.println(str2);
  		Map<String,String> map=new HashMap<String,String>();
  		map.put("bb","bb");
  		// 这里没有疑问，mapAdd中传递的map为对象，传递的是引用，操作后map指向了操作后堆中的map值
  		// 操作后，main方法中map中有两队值。
  		mapAdd(map);
  		for (String str : map.keySet()) {
  			System.out.println(str);
  		}
  	}
  

run this you will understand
------ For reference only --------------------------- ------------
then str points to a new abcdef, a method after , abcdef GC can be recycled , in fact, can not be here, the most critical is the String object caused by changes in value . If it is out of the ordinary new object , then passing , in a position to modify the original object value , such a method , the modified values ​​are still the original address is saved , it is passed by value , it can be understood , Daniel
- ----- For reference only ---------------------------------------
so str points to a new abcdef, a method after , abcdef GC can be recycled , in fact, the key here is the value of the String object is immutable caused . If it is out of the ordinary new object , then passing , in a position to modify the original object value , such a method , the modified values ​​are still the original address is saved , it is passed by value , it can be understood , Daniel  
This is the meaning
------ For reference only ------------------------------- --------

OK, thank you , pass it Results posted