Question

How inefficient is that:

List<Object> getList(){
    return new LinkedList<Object>();
}

void foo(){
    for(Object o: getList()){
       do something with o;
    }
}

in compare with that:

List<Object> getList(){
    return new LinkedList<Object>();
}

void foo(){
    List<Object>  os = getList();
    for(Object o: os){
       do something with o;
    }
}

Answer 1

There won’t be any perceptible difference.

I’ve compiled the following code:

import java.util.LinkedList;
import java.util.List;

public class Test1 {

    static List<Object> getList(){
        return new LinkedList<Object>();
    }

    void process(Object o) {}

    void foo1(){
        for(Object o: getList()){
            process(o);
        }
    }

    void foo2(){
        List<Object>  os = getList();
        for(Object o: os){
            process(o);
        }
    }
}

The bytecodes for foo1 and foo2 are as follows:

void foo1();
  Code:
   0:   invokestatic    #26; //Method getList:()Ljava/util/List;
   3:   invokeinterface #28,  1; //InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
   8:   astore_2
   9:   goto    24
   12:  aload_2
   13:  invokeinterface #34,  1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
   18:  astore_1
   19:  aload_0
   20:  aload_1
   21:  invokevirtual   #40; //Method process:(Ljava/lang/Object;)V
   24:  aload_2
   25:  invokeinterface #42,  1; //InterfaceMethod java/util/Iterator.hasNext:()Z
   30:  ifne    12
   33:  return

void foo2();
  Code:
   0:   invokestatic    #26; //Method getList:()Ljava/util/List;
   3:   astore_1
   4:   aload_1
   5:   invokeinterface #28,  1; //InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
   10:  astore_3
   11:  goto    26
   14:  aload_3
   15:  invokeinterface #34,  1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
   20:  astore_2
   21:  aload_0
   22:  aload_2
   23:  invokevirtual   #40; //Method process:(Ljava/lang/Object;)V
   26:  aload_3
   27:  invokeinterface #42,  1; //InterfaceMethod java/util/Iterator.hasNext:()Z
   32:  ifne    14
   35:  return

As you can see for yourself, the bytecodes for the two loops are identical. The only difference is that foo2 stores and loads the list reference in a local variable at the start.

One might argue that a better optimizing compiler may be able to eliminate os altogether, producing identical code for both functions.