For each loop in the null list

Gangnus :

If we do a foreach loop in an empty collection, simply no actions will be taken. But how should work the structure in the case, when the collection is null?

Collection<String> c=null;
....
for(String item:c){
   println(item);
}

I can run the code and see the null reference exception. But what should be the behaviour exactly? I can't find any manual page about that.

I know that the exception is thrown at the header, because the collection is null. But I want to know HOW the exception is thrown. It could be done by if, or by Objects.requireNonNull(), or in some other ways, or as it is shown in the David's answer.

davidxxx :

A enhanced for (or foreach) after compilation uses under the hood an Iterator to iterate on the collection as stated by the JLS :

The enhanced for statement is equivalent to a basic for statement of the form:

for (I #i = Expression.iterator(); #i.hasNext(); ) {
    {VariableModifier} TargetType Identifier =
        (TargetType) #i.next();
    Statement
}

You can check if yourself by looking the disassembled code of your loop after compilation (javap -c) :

  3: invokeinterface #2,  1            // InterfaceMethod java/util/Collection.iterator:()Ljava/util/Iterator;
  8: astore_2
  9: aload_2
 10: invokeinterface #3,  1            // InterfaceMethod java/util/Iterator.hasNext:()Z
 15: ifeq          38
 18: aload_2
 19: invokeinterface #4,  1            // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
 24: checkcast     #5                  // class java/lang/String
 27: astore_3
 28: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
 31: aload_3
 32: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
 35: goto          9

So, it is like if you had written :

Collection<String> c = null;
for (Iterator<String> iterator = c.iterator(); iterator.hasNext();) {
    String item = iterator.next();          
}

Invoking iterator() on null throws so a NullPointerException.

Guess you like

Origin http://10.200.1.11:23101/article/api/json?id=474289&siteId=1

For each loop in the null list

loop each element and perform its own action and then store it in list<String>

jquery each () loop

Decompiling for-each loop

The principle of java for each loop

jquery jumps out of the loop $().each

Strange for-each loop java

For each in java for list interface

python (18) - a list of list, for loop

Loop array (list sets List)

compare the items of each list at each index

Loop: Action List

for loop, slice list, tuple

Mybatis loop List

Loop Through Self List

List loop delete method

List seamless loop scrolling

Loop to delete elements in List

python list of null values

Each element is added to the list of labels

Hadoop each port service list

The progressive output elements of each list

list($name,$value)=each($_POST)

for each loop and traversed the way out of the cycle

2.3 UiPath For Each loop active introduction and use

[Base] Java Java enhanced for loop for each

Java_008 variables, ++a and a++, for each loop

Loop through each line of data in the text

Simple use example of MULE loop For Each

Simple use example of MULE loop For Each