RequireThis

Since Checkstyle 3.4

Description

Checks that references to instance variables and methods of the present object are explicitly of the form "this.varName" or "this.methodName(args)" and that those references don't rely on the default behavior when "this." is absent.

Warning: the Check is very controversial if 'validateOnlyOverlapping' option is set to 'false' and not that actual nowadays.

Rationale:

  1. The same notation/habit for C++ and Java (C++ have global methods, so having "this." do make sense in it to distinguish call of method of class instead of global).
  2. Non-IDE development (ease of refactoring, some clearness to distinguish static and non-static methods).

Notes

Limitations: Nothing is currently done about static variables or catch-blocks. Static methods invoked on a class name seem to be OK; both the class name and the method name have a DOT parent. Non-static methods invoked on either this or a variable name seem to be OK, likewise.

Properties

name description type default value since
checkFields Control whether to check references to fields. boolean true 3.4
checkMethods Control whether to check references to methods. boolean true 3.4
validateOnlyOverlapping Control whether to check only overlapping by variables or arguments. boolean true 6.17

Examples

To configure the default check: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="RequireThis"/>
  </module>
</module>

Example:

public class Test {
  private int a;
  private int b;
  private int c;

  public Test(int a) {
    // overlapping by constructor argument
    this.a = a;       // OK, this keyword used
    b = 0;            // OK, no overlap
    foo(5);           // OK
  }

  public void foo(int c) {
    // overlapping by method argument
    c = c;            // violation, reference to instance variable "c" requires "this"
  }
}

To configure the check for fields only: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="RequireThis">
      <property name="checkMethods" value="false"/>
    </module>
  </module>
</module>

Example:

public class Test {
  private int a;
  private int b;
  private int c;

  public Test(int a) {
    // overlapping by constructor argument
    this.a = a;       // OK, this keyword used
    b = 0;            // OK, no overlap
    foo(5);           // OK, no validation for methods
  }

  public void foo(int c) {
    // overlapping by method argument
    c = c;            // violation, reference to instance variable "c" requires "this"
  }
}

To configure the check for methods only: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="RequireThis">
      <property name="checkFields" value="false"/>
    </module>
  </module>
</module>

Example:

public class Test {
  private int a;
  private int b;
  private int c;

  public Test(int a) {
    // overlapping by constructor argument
    this.a = a;       // OK, no validation for fields
    b = 0;            // OK, no validation for fields
    foo(5);           // OK, no overlap
  }

  public void foo(int c) {
    // overlapping by method argument
    c = c;            // OK, no validation for fields
  }
}

Note that method call foo(5) does not raise a violation because methods cannot be overlapped in java.

To configure the check to validate for non-overlapping fields and methods: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="RequireThis">
      <property name="validateOnlyOverlapping" value="false"/>
    </module>
  </module>
</module>

Example:

public class Test {
  private int a;
  private int b;
  private int c;

  public Test(int a) {
    // overlapping by constructor argument
    this.a = a;       // OK, no validation for fields
    b = 0;            // violation, reference to instance variable "b" requires "this"
    foo(5);           // violation, method call "foo(5)" requires "this"
  }

  public void foo(int c) {
    // overlapping by method argument
    c = c;            // violation, reference to instance variable "c" requires "this"
  }
}

Please, be aware of the following logic, which is implemented in the check:

1) If you arrange 'this' in your code on your own, the check will not raise violation for variables which use 'this' to reference a class field, for example: 

public class C {
  private int scale;
  private int x;

  public void foo(int scale) {
    scale = this.scale;      // no violation

    if (scale > 0) {
      scale = -scale;        // no violation
    }
    x *= scale;
  }
}

2) If method parameter is returned from the method, the check will not raise violation for returned variable/parameter, for example: 

public class D {
  private String prefix;

  public String modifyPrefix(String prefix) {
    prefix = "^" + prefix + "$";  // no violation, because method parameter is returned
    return prefix;
  }
}

Example of Usage

Violation Messages

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker