The Java language provides two ways of passing parameters(参数)
①call by value:
In the method transfer,the actual parameters passes its value to the formal parameters,the formal parameter simply initializes(初始化) a temporary(临时的) storage unit with the value of the actual parameter.Therefore,although the formal parameters have the same value as the actual parameters,they have different storage units,so the modifcation(修改) of the formal parameters will not affect the value of the actual parameters.
②call by reference:
In the method tranfer,the content we passed is the object(it can also be seen as the address of the object),At this point,the formal parameter points to the same storage unit as the object of the actual parameter,so the modification of the formal parameter will affect the value of the actual parameter.
In Java language,the raw data types passes the parameter by value,while the wrapper class(包装类) passes the parameter by reference.
An example below describes the difference between call by value and call by reference:
public class Test {
public static void testPassParameter(StringBuffer ss1,int n) {
ss1.append("World");
n = 8;
}
public static void main(String[] args) {
int i = 1;
StringBuffer s1 = new StringBuffer("Hello");
testPassParameter(s1, i);
System.out.println(s1);
System.out.println(i);
}
}
The program run result is:
HelloWorld
1
call by reference is actually simillar to the passing pointer,taking the address of the object as a parameter.
As shown in the figure:
For comprehension,the 1 and hello stores are assumed(假设) to address 0x12345678 and 0xFFFFFF12.When calling method testPassParameter,the parameter is call by value since i is the base type,a copy of i is created which has the same value as i,assigning(赋值) this copy to n as a parameter,as the passed parameter,while StringBuffer is a class and therefore call by reference.Passing its reference(the address of Hello).In the inside of testPassParameter is modified the value of n which has no relation to i.However,in modifying ss1,the string that ss1 the address ss1 points to,since the form parameter ss1 points to the same storage space as actual parameter s1,the String that s1 points to after modifing ss1 is modified.
