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Code Analyzer for Java

There is a newer version: 8.6.0.37351

Why is this an issue?
Mutable objects are those whose state can be changed. For instance, an array is mutable, but a String is not. Private 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 the copy instead.
This rule checks that private arrays, collections and Dates are not stored or returned directly.
Noncompliant code example
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"};
  }
}

Compliant solution
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"};
  }
}

Resources

   CWE - CWE-374 - Passing Mutable Objects to an Untrusted Method 
   CWE - CWE-375 - Returning a Mutable Object to an Untrusted Caller 
   CERT, OBJ05-J. - Do not return references to private mutable class members 
   CERT, OBJ06-J. - Defensively copy mutable inputs and mutable internal components
  
   CERT, OBJ13-J. - Ensure that references to mutable objects are not exposed