[Java] The word passing by reference is bad!

Always stumble Java reference value passing

I often hear that passing by reference does not exist in Java. At first I thought "Eh", but I summarized the results of my research.

How to pass general method arguments

・ By value ・ Pass by reference -Pass by reference value

How to pass method arguments in Java

・ By value -Pass by reference value

• "Pass by reference" does not exist in Java. I will explain about "passing by value" and "passing by reference".

Example of passing by value

RefValHandOver.java

private static void editNumber(int arg){
	arg = 0;
	System.out.println("editNumber ： " + arg);
}
public static void main(String[] args) {
	int number = 100;
	editNumber(number);
	System.out.println( "main : " + number);
}

result

editNumber ： 0
main : 100

It is 0 in editNumber, but it remains 100 in main. If the argument is a primitive type, it will be passed by value.

Primitive type

There is no particular problem so far.

Example of passing by reference value

I will explain by classifying it into three patterns. ・ 1. When data is manipulated in the method ・ 2. Renew in the method. -3. Substitute null in the method.

RefValHandOver.java

// 1.When data is manipulated in a method
private static void methodAddArray(ArrayList<String> arg){
	arg.add("B");
	System.out.println("methodAddArray : " + arg.hashCode() + " : " + arg.toString());
}
//2.Renew in the method.
private static void methodNewArray(ArrayList<String> arg){
	arg = new ArrayList<String>();
	arg.add("C");
	System.out.println("methodNewArray : " +arg.hashCode() + " : " +  arg.toString());
}
//3.Assign null in the method.
private static void methodNullArray(ArrayList<String> arg){
	arg = null;
	System.out.println((arg != null) ? "methodNullArray : " + arg.hashCode() + " : " +  arg.toString() : "null");
}

public static void main(String[] args) {
	ArrayList<String> arrayList = new ArrayList<String>() {{add("A");}};
	// 1.When data is manipulated in a method
	methodAddArray(arrayList);
	System.out.println("main : " + arrayList.hashCode() + " : " +  arrayList.toString());

	//2.Renew in the method.
	methodNewArray(arrayList);
	System.out.println("main : " + arrayList.hashCode() + " : " +  arrayList.toString());

	//3.Assign null in the method.
	methodNullArray(arrayList);
	System.out.println((arrayList != null) ? "main : " + arrayList.hashCode() + " : " +  arrayList.toString() : "null");

}

result

// 1.When data is manipulated in a method
methodAddArray : 3042 : [A, B]
main : 3042 : [A, B]　　　　　　　//The reference destination has not changed

//2.Renew in the method.
methodNewArray : 98 : [C]
main : 3042 : [A, B]　　　　　　　//The reference destination has changed

//3.Assign null in the method.
methodNullArray : null
main : 3042 : [A, B]　　　　　　　//The reference destination has changed

Try to illustrate

I will explain

The focus is on whether you are ___assigning a new reference to the argument. When assigned, the "local variable in main" and the "argument in the method" will be different.

• In ___ passed by reference ___, which does not exist in Java, a few examples are reflected in the main local variable.

Summary

We have described ___passed by value ___ and ___ passed by reference value ___. Personally, I think it would be easier to understand if passing by reference value is ___ passing by reference address ___ </ span>.