PMD检查代码的一些规则

 

摘抄自：http://pmd.sourceforge.net/rules/optimizations.html

一、Optimization Rules:最佳规则

1、LocalVariableCouldBeFinal:A local variable assigned only once can be declared final.

2、MethodArgumentCouldBeFinal:A method argument that is never assigned can be declared final.

3、AvoidInstantiatingObjectsInLoops:Detects when a new object is created inside a loop

public class Something {
  public static void main( String as[] ) {  
    for (int i = 0; i < 10; i++) {
      Foo f = new Foo(); //Avoid this whenever you can it's really expensive
    }
  }
}

5、UseArrayListInsteadOfVector:ArrayList is a much better Collection implementation than Vector.

6、UseArraysAsList:The java.util.Arrays class has a "asList" method that should be used when you want to create a new List from an array of objects. It is faster than executing a loop to copy all the elements of the array one by one.

 

   public void foo(Integer[] ints) {
    // could just use Arrays.asList(ints)
     List l= new ArrayList(10);
     for (int i=0; i< 100; i++) {
      l.add(ints[i]);
     }
     for (int i=0; i< 100; i++) {
      l.add(a[i].toString()); // won't trigger the rule
     }
    }

 

 7、AvoidArrayLoops:Instead of copying data between two arrays, use System.arraycopy method.

 

public void bar() {
  int[] a = new int[10];
  int[] b = new int[10];
  for (int i=0;i<10;i++) {
   b[i]=a[i];
  }
 }

// this will trigger the rule
   for (int i=0;i<10;i++) {
      b[i]=a[c[i]];
    }
}     

8 、AddEmptyStirng:Finds empty string literals which are being added. This is an inefficient(无效) way to convert any type to a String.

 

 

 String s = "" + 123; // bad 
 String t = Integer.toString(456); // ok 

二、Basic Rules:The Basic Ruleset contains a collection of good practices which everyone should follow.

1、EmptyCatchBlock:

public void doSomething() {
  try {
    FileInputStream fis = new FileInputStream("/tmp/bugger");
  } catch (IOException ioe) {
      // not good
  }

2、JumbledIncrementer:Avoid jumbled loop incrementers - it's usually a mistake, and it's confusing even if it's what's intended.

for (int i = 0; i < 10; i++) {
    for (int k = 0; k < 20; i++) {
     System.out.println("Hello");
    }
}

 3、UnnecessaryConventionTemporary:Avoid unnecessary temporaries when converting primitives to Strings

// this wastes an object
  String foo = new Integer(x).toString();
// this is better
  return Integer.toString(x);

  3、DoubleCheckingLocking:Partially(部分的) created objects can be returned by the Double Checked Locking pattern when used in Java. An optimizing(最佳的) JRE may assign a reference to the baz variable before it creates the object the reference is intended to point to

 Object baz;
  Object bar() {
    if(baz == null) { //baz may be non-null yet not fully created
      synchronized(this){
        if(baz == null){
          baz = new Object();
        }
      }
    }
    return baz;
  }

 4、ReturnFromFinalBlock:Avoid returning from a finally block - this can discard exceptions.

 5、UnnecessaryReturn

 public void bar() {
  int x = 42;
  return;
 }

6、UnnecessaryReturn

7、UnConditionIfStatement:Do not use "if" statements that are always true or always false.

8、BooleanInstantiation:Avoid instantiating Boolean objects; you can reference Boolean.TRUE, Boolean.FALSE, or call Boolean.valueOf() instead.

9、UnnnessaryFinalModifier:

public final class Foo {
    // This final modifier is not necessary, since the class is final
    // and thus, all methods are final
    private final void foo() {
    }
}

 10、CollapsiableIfStament:Sometimes two 'if' statements can be consolidated(合并) by separating their conditions with a boolean short-circuit operator(短路&&).

void bar() {
  if (x) {
   if (y) {
    // do stuff
   }
  }
 }

11、ClassCastExceptionWithToArray:

public static void main(String[] args) {
        Collection c=new ArrayList();
        Integer obj=new Integer(1);
        c.add(obj);

        // this would trigger the rule (and throw a ClassCastException if executed)
        Integer[] a=(Integer [])c.toArray();

        // this wouldn't trigger the rule
        Integer[] b=(Integer [])c.toArray(new Integer[c.size()]);
    }

12、BrokenNullCheck:The null check is broken since it will throw a NullPointerException itself. It is likely that you used || instead of && or vice versa(反之亦然).

// should be &&
  if (string!=null || !string.equals(""))
    return string;
  // should be ||
  if (string==null && string.equals(""))
    return string;

 13、AvoidUsingOctalValue:Integer literals should not start with zero. Zero means that the rest of literal will be interpreted(理解) as an octal value(八进制).

public class Foo {
     int i = 012; // set i with 10 not 12
     int j = 010; // set j with 8 not 10
     k = i * j; // set k with 80 not 120
}