String is passed by “reference” in Java

本文通过对比Java和C++中字符串修改行为的差异,深入解析了Java的传值机制。揭示了变量存储的是引用而非引用本身的概念,并提供了正确的理解和解决方法。

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This is a classic question of Java. Many similar questions have been asked on stackoverflow, and there are a lot of incorrect/incomplete answers. The question is simple if you don’t think too much. But it could be very confusing, if you give more thought to it.

1. A code fragment that is interesting & confusing

public static void main(String[] args) {
	String x = new String("ab");
	change(x);
	System.out.println(x);
}
 
public static void change(String x) {
	x = "cd";
}

It prints “ab”.

In C++, the code is as follows:

void change(string &x) {
    x = "cd";
}
 
int main(){
    string x = "ab";
    change(x);
    cout << x << endl;
}

it prints “cd”.

2. Common confusing questions

x stores the reference which points to the “ab” string in the heap. So when x is passed as a parameter to the change() method, it still points to the “ab” in the heap like the following:

string-pass-by-reference

Because java is pass-by-value, the value of x is the reference to “ab”. When the method change() gets invoked, it creates a new “cd” object, and x now is pointing to “cd” like the following:

string-pass-by-reference 2

It seems to be a pretty reasonable explanation. They are clear that Java is always pass-by-value. But what is wrong here?

3. What the code really does?

The explanation above has several mistakes. To understand this easily, it is a good idea to briefly walk though the whole process.

When the string “ab” is created, Java allocates the amount of memory required to store the string object. Then, the object is assigned to variable x, the variable is actually assigned a reference to the object. This reference is the address of the memory location where the object is stored.

The variable x contains a reference to the string object. x is not a reference itself! It is a variable that stores a reference(memory address).

Java is pass-by-value ONLY. When x is passed to the change() method, a copy of value of x (a reference) is passed. The method change() creates another object “cd” and it has a different reference. It is the variable x that changes its reference(to “cd”), not the reference itself.

The following diagram shows what it really does.
string-pass-by-reference 3

4. The wrong explanation

The problem raised from the first code fragment is nothing related with string immutability. Even if String is replaced with StringBuilder, the result is still the same. The key point is that variable stores the reference, but is not the reference itself!

5. Solution to this problem

If we really need to change the value of the object. First of all, the object should be changeable, e.g.,StringBuilder. Secondly, we need to make sure that there is no new object created and assigned to the parameter variable, because Java is passing-by-value only.

public static void main(String[] args) {
	StringBuilder x = new StringBuilder("ab");
	change(x);
	System.out.println(x);
}
 
public static void change(StringBuilder x) {
	x.delete(0, 2).append("cd");
}


项目创建后有如下错误: AndroidManifest.xml文件的错误: 无法解析符号 '@style/Theme.FloatingSearchApp' READ_EXTERNAL_STORAGE is deprecated (and is not granted) when targeting Android 13+. If you need to query or interact with MediaStore or media files on the shared storage, you should instead use one or more new storage permissions: `READ_MEDIA_IMAGES`, `READ_MEDIA_VIDEO` or `READ_MEDIA_AUDIO`. Redundant label can be removed DatabaseHelper.kt文件的错误: 应为 <database name> 或 <defined table name>,得到 '$' Value must be ≥ 0 but `getColumnIndex` can be -1 Parameter 'question' is never used FloatingWindowService.kt文件的错误: Avoid passing `null` as the view root (needed to resolve layout parameters on the inflated layout's root element) 'TYPE_PHONE: Int' is deprecated. Deprecated in Java `onTouch` should call `View#performClick` when a click is detected Do not concatenate text displayed with `setText`. Use resource string with placeholders. String literal in `setText` can not be translated. Use Android resources instead. Variable 'answer' is never used Do not concatenate text displayed with `setText`. Use resource string with placeholders. String literal in `setText` can not be translated. Use Android resources instead. MainActivity.kt文件的错误: Unresolved reference: activity_main This declaration overrides deprecated member but not marked as deprecated itself. Please add @Deprecated annotation or suppress. See https://youtrack.jetbrains.com/issue/KT-47902 for details TextRecognizer.kt文件的错误: No value passed for parameter 'options' 'getter for defaultDisplay: Display!' is deprecated. Deprecated in Java 'getMetrics(DisplayMetrics!): Unit' is deprecated. Deprecated in Java Parameter 'context' is never used floating_window.xml文件的错误: Hardcoded string "问题将显示在这里", should use `@string` resource Hardcoded string "答案将显示在这里", should use `@string` resource
06-22
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