sebthom/jstuff

Calling System.gc() or Runtime.getRuntime().gc() is a bad idea for a simple reason: there is no way to know exactly what will be done under the hood by the JVM because the behavior will depend on its vendor, version and options:

• Will the whole application be frozen during the call?
• Is the -XX:DisableExplicitGC option activated?
• Will the JVM simply ignore the call?
• ...

Like for System.gc(), there is no reason to manually call runFinalization() to force the call of finalization methods of any objects pending finalization.

An application relying on these unpredictable methods is also unpredictable and therefore broken. The task of running the garbage collector and calling finalize() methods should be left exclusively to the JVM.

Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.

On the other hand, constants can be referenced from many places, but only need to be updated in a single place.

Noncompliant Code Example

With the default threshold of 3:

public void run() {
  prepare("action1");                              // Noncompliant - "action1" is duplicated 3 times
  execute("action1");
  release("action1");
}

@SuppressWarning("all")                            // Compliant - annotations are excluded
private void method1() { /* ... */ }
@SuppressWarning("all")
private void method2() { /* ... */ }

public String method3(String a) {
  System.out.println("'" + a + "'");               // Compliant - literal "'" has less than 5 characters and is excluded
  return "";                                       // Compliant - literal "" has less than 5 characters and is excluded
}

Compliant Solution

private static final String ACTION_1 = "action1";  // Compliant

public void run() {
  prepare(ACTION_1);                               // Compliant
  execute(ACTION_1);
  release(ACTION_1);
}

Exceptions

To prevent generating some false-positives, literals having less than 5 characters are excluded.

The transitive property says that if a == b and b == c, then a == c. In such cases, there's no point in assigning a to c or vice versa because they're already equivalent.

This rule raises an issue when an assignment is useless because the assigned-to variable already holds the value on all execution paths.

Noncompliant Code Example

a = b;
c = a;
b = c; // Noncompliant: c and b are already the same

Compliant Solution

a = b;
c = a;

Fields in a Serializable class must themselves be either Serializable or transient even if the class is never explicitly serialized or deserialized. For instance, under load, most J2EE application frameworks flush objects to disk, and an allegedly Serializable object with non-transient, non-serializable data members could cause program crashes, and open the door to attackers. In general a Serializable class is expected to fulfil its contract and not have an unexpected behaviour when an instance is serialized.

This rule raises an issue on non-Serializable fields, and on collection fields when they are not private (because they could be assigned non-Serializable values externally), and when they are assigned non-Serializable types within the class.

Noncompliant Code Example

public class Address {
  //...
}

public class Person implements Serializable {
  private static final long serialVersionUID = 1905122041950251207L;

  private String name;
  private Address address;  // Noncompliant; Address isn't serializable
}

Compliant Solution

public class Address implements Serializable {
  private static final long serialVersionUID = 2405172041950251807L;
}

public class Person implements Serializable {
  private static final long serialVersionUID = 1905122041950251207L;

  private String name;
  private Address address;
}

Exceptions

The alternative to making all members serializable or transient is to implement special methods which take on the responsibility of properly serializing and de-serializing the object. This rule ignores classes which implement the following methods:

 private void writeObject(java.io.ObjectOutputStream out)
     throws IOException
 private void readObject(java.io.ObjectInputStream in)
     throws IOException, ClassNotFoundException;

See

• MITRE, CWE-594 - Saving Unserializable Objects to Disk
• Oracle Java 6, Serializable
• Oracle Java 7, Serializable

Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.

On the other hand, constants can be referenced from many places, but only need to be updated in a single place.

Noncompliant Code Example

With the default threshold of 3:

public void run() {
  prepare("action1");                              // Noncompliant - "action1" is duplicated 3 times
  execute("action1");
  release("action1");
}

@SuppressWarning("all")                            // Compliant - annotations are excluded
private void method1() { /* ... */ }
@SuppressWarning("all")
private void method2() { /* ... */ }

public String method3(String a) {
  System.out.println("'" + a + "'");               // Compliant - literal "'" has less than 5 characters and is excluded
  return "";                                       // Compliant - literal "" has less than 5 characters and is excluded
}

Compliant Solution

private static final String ACTION_1 = "action1";  // Compliant

public void run() {
  prepare(ACTION_1);                               // Compliant
  execute(ACTION_1);
  release(ACTION_1);
}

Exceptions

To prevent generating some false-positives, literals having less than 5 characters are excluded.

Classes in the sun.* or com.sun.* packages are considered implementation details, and are not part of the Java API.

They can cause problems when moving to new versions of Java because there is no backwards compatibility guarantee. Similarly, they can cause problems when moving to a different Java vendor, such as OpenJDK.

Such classes are almost always wrapped by Java API classes that should be used instead.

Noncompliant Code Example

import com.sun.jna.Native;     // Noncompliant
import sun.misc.BASE64Encoder; // Noncompliant

Utility classes, which are collections of static members, are not meant to be instantiated. Even abstract utility classes, which can be extended, should not have public constructors.

Java adds an implicit public constructor to every class which does not define at least one explicitly. Hence, at least one non-public constructor should be defined.

Noncompliant Code Example

class StringUtils { // Noncompliant

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}

Compliant Solution

class StringUtils { // Compliant

  private StringUtils() {
    throw new IllegalStateException("Utility class");
  }

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}

Exceptions

When class contains public static void main(String[] args) method it is not considered as utility class and will be ignored by this rule.

Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.

On the other hand, constants can be referenced from many places, but only need to be updated in a single place.

Noncompliant Code Example

With the default threshold of 3:

public void run() {
  prepare("action1");                              // Noncompliant - "action1" is duplicated 3 times
  execute("action1");
  release("action1");
}

@SuppressWarning("all")                            // Compliant - annotations are excluded
private void method1() { /* ... */ }
@SuppressWarning("all")
private void method2() { /* ... */ }

public String method3(String a) {
  System.out.println("'" + a + "'");               // Compliant - literal "'" has less than 5 characters and is excluded
  return "";                                       // Compliant - literal "" has less than 5 characters and is excluded
}

Compliant Solution

private static final String ACTION_1 = "action1";  // Compliant

public void run() {
  prepare(ACTION_1);                               // Compliant
  execute(ACTION_1);
  release(ACTION_1);
}

Exceptions

To prevent generating some false-positives, literals having less than 5 characters are excluded.

Utility classes, which are collections of static members, are not meant to be instantiated. Even abstract utility classes, which can be extended, should not have public constructors.

Java adds an implicit public constructor to every class which does not define at least one explicitly. Hence, at least one non-public constructor should be defined.

Noncompliant Code Example

class StringUtils { // Noncompliant

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}

Compliant Solution

class StringUtils { // Compliant

  private StringUtils() {
    throw new IllegalStateException("Utility class");
  }

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}

Exceptions

When class contains public static void main(String[] args) method it is not considered as utility class and will be ignored by this rule.

Overriding or shadowing a variable declared in an outer scope can strongly impact the readability, and therefore the maintainability, of a piece of code. Further, it could lead maintainers to introduce bugs because they think they're using one variable but are really using another.

Noncompliant Code Example

class Foo {
  public int myField;

  public void doSomething() {
    int myField = 0;
    ...
  }
}

See

• CERT, DCL01-C. - Do not reuse variable names in subscopes
• CERT, DCL51-J. - Do not shadow or obscure identifiers in subscopes

The transitive property says that if a == b and b == c, then a == c. In such cases, there's no point in assigning a to c or vice versa because they're already equivalent.

This rule raises an issue when an assignment is useless because the assigned-to variable already holds the value on all execution paths.

Noncompliant Code Example

a = b;
c = a;
b = c; // Noncompliant: c and b are already the same

Compliant Solution

a = b;
c = a;

Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.

On the other hand, constants can be referenced from many places, but only need to be updated in a single place.

Noncompliant Code Example

With the default threshold of 3:

public void run() {
  prepare("action1");                              // Noncompliant - "action1" is duplicated 3 times
  execute("action1");
  release("action1");
}

@SuppressWarning("all")                            // Compliant - annotations are excluded
private void method1() { /* ... */ }
@SuppressWarning("all")
private void method2() { /* ... */ }

public String method3(String a) {
  System.out.println("'" + a + "'");               // Compliant - literal "'" has less than 5 characters and is excluded
  return "";                                       // Compliant - literal "" has less than 5 characters and is excluded
}

Compliant Solution

private static final String ACTION_1 = "action1";  // Compliant

public void run() {
  prepare(ACTION_1);                               // Compliant
  execute(ACTION_1);
  release(ACTION_1);
}

Exceptions

To prevent generating some false-positives, literals having less than 5 characters are excluded.

Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.

On the other hand, constants can be referenced from many places, but only need to be updated in a single place.

Noncompliant Code Example

With the default threshold of 3:

public void run() {
  prepare("action1");                              // Noncompliant - "action1" is duplicated 3 times
  execute("action1");
  release("action1");
}

@SuppressWarning("all")                            // Compliant - annotations are excluded
private void method1() { /* ... */ }
@SuppressWarning("all")
private void method2() { /* ... */ }

public String method3(String a) {
  System.out.println("'" + a + "'");               // Compliant - literal "'" has less than 5 characters and is excluded
  return "";                                       // Compliant - literal "" has less than 5 characters and is excluded
}

Compliant Solution

private static final String ACTION_1 = "action1";  // Compliant

public void run() {
  prepare(ACTION_1);                               // Compliant
  execute(ACTION_1);
  release(ACTION_1);
}

Exceptions

To prevent generating some false-positives, literals having less than 5 characters are excluded.

Cognitive Complexity is a measure of how hard the control flow of a method is to understand. Methods with high Cognitive Complexity will be difficult to maintain.

See

• Cognitive Complexity

Overriding or shadowing a variable declared in an outer scope can strongly impact the readability, and therefore the maintainability, of a piece of code. Further, it could lead maintainers to introduce bugs because they think they're using one variable but are really using another.

Noncompliant Code Example

class Foo {
  public int myField;

  public void doSomething() {
    int myField = 0;
    ...
  }
}

See

• CERT, DCL01-C. - Do not reuse variable names in subscopes
• CERT, DCL51-J. - Do not shadow or obscure identifiers in subscopes

Overriding or shadowing a variable declared in an outer scope can strongly impact the readability, and therefore the maintainability, of a piece of code. Further, it could lead maintainers to introduce bugs because they think they're using one variable but are really using another.

Noncompliant Code Example

class Foo {
  public int myField;

  public void doSomething() {
    int myField = 0;
    ...
  }
}

See

• CERT, DCL01-C. - Do not reuse variable names in subscopes
• CERT, DCL51-J. - Do not shadow or obscure identifiers in subscopes

Classes in the sun.* or com.sun.* packages are considered implementation details, and are not part of the Java API.

They can cause problems when moving to new versions of Java because there is no backwards compatibility guarantee. Similarly, they can cause problems when moving to a different Java vendor, such as OpenJDK.

Such classes are almost always wrapped by Java API classes that should be used instead.

Noncompliant Code Example

import com.sun.jna.Native;     // Noncompliant
import sun.misc.BASE64Encoder; // Noncompliant

Classes in the sun.* or com.sun.* packages are considered implementation details, and are not part of the Java API.

They can cause problems when moving to new versions of Java because there is no backwards compatibility guarantee. Similarly, they can cause problems when moving to a different Java vendor, such as OpenJDK.

Such classes are almost always wrapped by Java API classes that should be used instead.

Noncompliant Code Example

import com.sun.jna.Native;     // Noncompliant
import sun.misc.BASE64Encoder; // Noncompliant

Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.

On the other hand, constants can be referenced from many places, but only need to be updated in a single place.

Noncompliant Code Example

With the default threshold of 3:

public void run() {
  prepare("action1");                              // Noncompliant - "action1" is duplicated 3 times
  execute("action1");
  release("action1");
}

@SuppressWarning("all")                            // Compliant - annotations are excluded
private void method1() { /* ... */ }
@SuppressWarning("all")
private void method2() { /* ... */ }

public String method3(String a) {
  System.out.println("'" + a + "'");               // Compliant - literal "'" has less than 5 characters and is excluded
  return "";                                       // Compliant - literal "" has less than 5 characters and is excluded
}

Compliant Solution

private static final String ACTION_1 = "action1";  // Compliant

public void run() {
  prepare(ACTION_1);                               // Compliant
  execute(ACTION_1);
  release(ACTION_1);
}

Exceptions

To prevent generating some false-positives, literals having less than 5 characters are excluded.

It is highly recommended not to use wildcard types as return types. Because the type inference rules are fairly complex it is unlikely the user of that API will know how to use it correctly.

Let's take the example of method returning a "List<? extends Animal>". Is it possible on this list to add a Dog, a Cat, ... we simply don't know. And neither does the compiler, which is why it will not allow such a direct use. The use of wildcard types should be limited to method parameters.

This rule raises an issue when a method returns a wildcard type.

Noncompliant Code Example

List<? extends Animal> getAnimals(){...}

Compliant Solution

List<Animal> getAnimals(){...}

or

List<Dog> getAnimals(){...}