Lambda Expressions explanation please

[Alexander Zotz]

Code:

List<Integer> names = Arrays.asList(1, 2, 3); 
names.forEach(x->x=x+1);
names.forEach(System.out::println);

The expression x->x=x+1; doesn't actually change the elements in the list.

Why does following code changes the objects in list: (assume Student have two fields name and age and properly getter/setter and overriden toString method )

List<Student> students= Arrays.asList(
  new Student("paul",20),
  new Student("andrea",31)
); 
students.forEach(x->x=x.setAge = 40);
students.forEach(System.out::println);

[Campbell Ritchie]

Added to our λs forum.

Alexander Zotz wrote:. . . x->x=x+1; doesn't actually change the elements in the list.

No. If you look at ArrayList#forEach(), you find it is defined elsewhere, and it passes a copy of the result got from the Iterator to the Consumer. That Consumer consumes what has been passed; all it does is replace it with a new value. Just as in any other pass by value situation, the assignments are not reflected back to the original source. This link says that forEach() behaves just like an enhanced for loop, also called a for‑each loop (hence the name forEach()), and a for‑each loop does not pass assignments back to its source.

Why does following code changes the objects in list . . .

It does not change the objects in the List. Because your List contains mutable objects, it is possible to change those objects' state (i.e. the age field) without changing which objects are in the List.