Lambda ‘special void-compatibility rule’ – statement expression

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The term “statement expression” or “expression statement” refers to expressions that are also allowed to be used as a statement. They are described in the Java Language Specification, §14.8. Expression Statements.

They include:

  • Method Invocations
  • Assignments
  • Increment/Decrement expressions
  • Class Instance Creation expressions

So other examples are:

Consumer<String> b = s -> counter++;
Function<String,Integer> f = s -> counter++;

or

Consumer<String> b = s -> new BigDecimal(s);
Function<String,BigDecimal> f = s -> new BigDecimal(s);

As a rule of thumb, a lambda expression of the form x -> expression is only legal for a Consumer (or void function type in general), if x -> { expression; } would be legal too.

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