Mutable objects are those whose state can be changed. For instance, an array is mutable, but a String is not. Mutable class members should never be returned to a caller or accepted and stored directly. Doing so leaves you vulnerable to unexpected changes in your class state.

Instead use an unmodifiable Collection

(via Collections.unmodifiableCollection, Collections.unmodifiableList, ...) or make a copy of the mutable object, and store or return copy instead.

This rule checks that arrays, collections and Dates are not stored or returned directly.

可變對象是那些狀態可以改變的對象。

例如，數組是可變的，但String不是。 永遠不應將可變類成員返回給調用者或直接接受和存儲。 這樣做會使您容易受到類狀態的意外更改的影響。

而是使用不可修改的Collection（通過Collections.unmodifiableCollection，Collections.unmodifiableList，...）或制作可變對象的副本，然後存儲或返回副本。

此規則檢查是否未直接存儲或返回數組，集合和日期。

不合格的代碼如下：

class A {
  private String [] strings;

  public A () {
    strings = new String[]{"first", "second"};
  }

  public String [] getStrings() {
    return
 
 strings; // Noncompliant
  }

  public void setStrings(String [] strings) {
    this.strings = strings;  // Noncompliant
  }
}

public class B {

  private A a = new A();  // At this point a.strings = {"first", "second"};

  public void wreakHavoc() {
    a.getStrings()[0] = "yellow";  // a.strings = {"yellow", "second"};
  }
}

合格代碼解決方案

class A {
  private String [] strings;

  public A () {
    strings = new String[]{"first", "second"};
  }

  public String [] getStrings() {
    return strings.clone();
  }

  public void setStrings(String [] strings) {
    this.strings = strings.clone();
  }
}

public class B {

  private A a = new A();  // At this point a.strings = {"first", "second"};

  public void wreakHavoc() {
    a.getStrings()[0] = "yellow";  // a.strings = {"first", "second"};
  }
}

不應直接存儲或返回可變成員 Mutable members should not be stored or returned directly