[Java] Summary of how to abbreviate lambda expressions

Here's a summary of how to abbreviate lambda expressions. It will be asked in exams such as Java Silver, so I hope it will help the examinees. Finally, I will summarize how to remember.

If you don't understand the lambda expression in the first place, please click here. Understanding Java 8 Lambda Expressions

First, the basic form of the lambda type.

`Uninflected word`


Predicate<String> javaChecker = (String s) -> { return s.equals("Java"); };

Let's see what can be omitted from here.

Argument type

First, the argument types are optional.

Predicate<String> javaChecker = (s) -> { return s.equals("Java"); };

However, if there are multiple arguments, it is not possible to omit only one.

//Compile error
BiFunction<Integer, Integer, Integer> adder = (a, Integer b) -> { return a + b; };

Argument ()

The argument () can also be omitted.

Predicate<String> javaChecker = s -> { return s.equals("Java"); };

However, there are two conditions. First, if you specify a type, you cannot omit it.

//Compile error
Predicate<String> javaChecker = String s -> { return s.equals("Java"); };

Second, it cannot be omitted even if there are multiple arguments.

//Compile error
BiFunction<Integer, Integer, Integer> adder = a, b -> { return a + b; };

I think it's good to remember that () is optional only when the argument is one word.

Method {}

The method {} can also be omitted.

Consumer<String> buyer = goods -> System.out.println(goods + "I bought");

There are also conditions here. First, it cannot be omitted if there are two or more sentences.

//Compile error
Consumer<String> buyer = goods -> String message = "I," + goods;
                                  System.out.println(message + "I bought");

If you write return next, you cannot omit it.

//Compile error
Predicate<String> javaChecker = s -> return s.equals("Java");

However, if you omit return, you can execute it normally.

Predicate<String> javaChecker = s -> s.equals("Java");

Remember that {} can be omitted only when the process is one statement, and if there is a return value, the return can be omitted as a set.

return As mentioned above, return can be omitted, but it cannot be omitted if the method {} is left.

//Compile error
Predicate<String> javaChecker = s ->{ s.equals("Java"); };

Even if you want to omit the return, it will be a set with the omission of {}.

Summary

The summary is as follows.

Omitted part	conditions
Argument type	全Argument typeを省略すること
Of the argument()	Only for one word
Of the method{}	Only for one statement If there is a return value, set with omission of return
return	Of the method{}Omission and set