https://docs.oracle.com/javase/specs/jls/se11/html/jls-14.html#jls-14.8

https://stackoverflow.com/questions/41482574/lambda-special-void-compatibility-rule-statement-expression

Certain kinds of expressions may be used as statements by following them with semicolons.

• ExpressionStatement:

  StatementExpression ;
  

• StatementExpression:
  • Assignment
  • PreIncrementExpression
  • PreDecrementExpression
  • PostIncrementExpression
  • PostDecrementExpression
  • MethodInvocation
  • ClassInstanceCreationExpression

An expression statement is executed by evaluating the expression; if the expression has a value, the value is discarded.

• assigning a function (boolean add(E e)) to Consumer

  var list = new LinkedList<String>();
  Consumer<String> add = list::add;
  
  add.accept("a");
  
  assertThat(list, hasSize(1));
  

• function passed as a Consumer

  var source = List.of(1, 2, 3);
  var target = new LinkedList<Integer>();
  
  source.forEach(target::add);
  
  assertThat(target, is(source));
  

• Consumer that increment the field

  private int x = 5; // field
  
  // then in code
  Consumer<String> increment = s -> x++;
  
  increment.accept("a");
  increment.accept("a");
  increment.accept("a");
  increment.accept("a");
  
  assertThat(x, is(9));
  

• class creation

  Consumer<Integer> create = X::new;
  
  create.accept(1);
  create.accept(1);
  
  assertThat(X.counter, is(2));
  

  where

  class X {
      static int counter;
  
      X(int i) {
          counter++;
      }
  }