26 List of Mutators
26.1 Conditionals Boundary Mutator (CONDITIONALS_BOUNDARY)
The conditionals boundary mutator replaces the relational operators <, <=, >, >=
with their boundary counterpart as per the table below.
| Original conditional | Mutated conditional |
|---|---|
| < | <= |
| <= | < |
| > | >= |
| >= | > |
{:.table }
For example
if (a < b) {
// do something
}will be mutated to
if (a <= b) {
// do something
}26.2 Increments Mutator (INCREMENTS)
The increments mutator will mutate increments, decrements and assignment increments and decrements of local variables (stack variables). It will replace increments with decrements and vice versa.
For example
public int method(int i) {
i++;
return i;
}will be mutated to
public int method(int i) {
i--;
return i;
}Please note that the increments mutator will be applied to increments of local variables only. Increments and decrements of member variables will be covered by the Math Mutator.
26.3 Invert Negatives Mutator (INVERT_NEGS)
The invert negatives mutator inverts negation of integer and floating point numbers. For example
public float negate(final float i) {
return -i;
}will be mutated to
public float negate(final float i) {
return i;
}26.4 Math Mutator (MATH)
The math mutator replaces binary arithmetic operations for either integer or floating-point arithmetic with another operation. The replacements will be selected according to the table below.
| Original conditional | Mutated conditional |
|---|---|
| + | - |
| - | + |
| * | / |
| / | * |
| % | * |
| & | | |
| | | & |
| ^ | & |
| << | >> |
| >> | << |
| >>> | << |
{:.table}
For example
int a = b + c;will be mutated to
int a = b - c;Keep in mind that the + operator on Strings as in
String a = "foo" + "bar";is not a mathematical operator but a string concatenation and will be replaced by the compiler with something like
String a = new StringBuilder("foo").append("bar").toString();Please note that the compiler will also use binary arithmetic operations for increments, decrements and assignment increments and decrements of non-local variables (member variables) although a special iinc opcode for increments exists. This special opcode is restricted to local variables (also called stack variables) and cannot be used for member variables. That means the math mutator will also mutate
public class A {
private int i;
public void foo() {
this.i++;
}
}to
public class A {
private int i;
public void foo() {
this.i = this.i - 1;
}
}See the Increments Mutator for details.
26.5 Negate Conditionals Mutator (NEGATE_CONDITIONALS)
The negate conditionals mutator will mutate all conditionals found according to the replacement table below.
| Original conditional | Mutated conditional |
|---|---|
| == | != |
| != | == |
| <= | > |
| >= | < |
| < | >= |
| > | <= |
{:.table }
For example
if (a == b) {
// do something
}will be mutated to
if (a != b) {
// do something
}This mutator overlaps to a degree with the conditionals boundary mutator, but is less stable i.e these mutations are generally easier for a test suite to detect.
26.6 Return Values Mutator (RETURN_VALS)
This mutator has been superseded by the new returns mutator set. See Empty returns, False returns, True returns, Null returns and Primitive returns.
The return values mutator mutates the return values of method calls. Depending on the return type of the method another mutation is used.4
| Return Type | Mutation |
|---|---|
boolean |
replace the unmutated return value true with false and replace the unmutated return value false with true |
int byte short |
if the unmutated return value is 0 return 1, otherwise mutate to return value 0 |
long |
replace the unmutated return value x with the result of x+1 |
float double |
replace the unmutated return value x with the result of -(x+1.0) if x is not NAN and replace NAN with 0 |
Object |
replace non-null return values with null and throw a java.lang.RuntimeException if the unmutated method would return null |
{:.table}
For example
public Object foo() {
return new Object();
}will be mutated to
public Object foo() {
new Object();
return null;
}Please note that constructor calls are not considered void method calls. See the Constructor Call Mutator for mutations of constructors or the Non Void Method Call Mutator for mutations of non void methods.
26.7 Void Method Call Mutator (VOID_METHOD_CALLS)
The void method call mutator removes method calls to void methods. For example
public void someVoidMethod(int i) {
// does something
}
public int foo() {
int i = 5;
someVoidMethod(i);
return i;
}will be mutated to
public void someVoidMethod(int i) {
// does something
}
public int foo() {
int i = 5;
return i;
}26.8 Empty returns Mutator (EMPTY_RETURNS)
Replaces return values with an ‘empty’ value for that type as follows
- java.lang.String -> “”
- java.util.Optional -> Optional.empty()
- java.util.List -> Collections.emptyList()
- java.util.Collection -> Collections.emptyList()
- java.util.Set -> Collections.emptySet()
- java.lang.Integer -> 0
- java.lang.Short -> 0
- java.lang.Long -> 0
- java.lang.Character -> 0
- java.lang.Float -> 0
- java.lang.Double -> 0
Pitest will filter out equivalent mutations to methods that are already hard coded to return the empty value.
26.9 False returns Mutator (FALSE_RETURNS)
Replaces primitive and boxed boolean return values with false.
Pitest will filter out equivalent mutations to methods that are already hard coded to return false.
26.10 True returns Mutator (TRUE_RETURNS)
Replaces primitive and boxed boolean return values with true.
Pitest will filter out equivalent mutations to methods that are already hard coded to return true.
26.11 Null returns Mutator (NULL_RETURNS)
Replaces return values with null. Methods that can be mutated by the EMPTY_RETURNS mutator or that are directly annotated with NotNull will not be mutated.
Pitest will filter out equivalent mutations to methods that are already hard coded to return null.
26.12 Primitive returns Mutator (PRIMITIVE_RETURNS)
Replaces int, short, long, char, float and double return values with 0.
Pitest will filter out equivalent mutations to methods that are already hard coded to return 0.
26.13 Constructor Call Mutator (CONSTRUCTOR_CALLS)
Optional mutator that replaces constructor calls with null values. For example
public Object foo() {
Object o = new Object();
return o;
}will be mutated to
public Object foo() {
Object o = null;
return o;
}Please note that this mutation is fairly unstable and likely to cause NullPointerExceptions even with weak test suites.
This mutator does not affect non constructor method calls. See Void Method Call Mutator for mutations of void methods and Non Void Method Call Mutator for mutations of non void methods.