How to correctly understand and apply the 'Class::Instance Method' method reference in Java?

Deep understanding of Java method references: Class::Instance Methods

Java method references bring simplicity and readability to the code, especially the form of类::实例方法. This article will explain its working method in detail and use examples to help you better understand and apply it.

Background introduction

Consider the following code snippet:

 ArrayList<string> list = new ArrayList();
Collections.addAll(list, "a", "b", "c");
// Convert collection elements to uppercase list.stream()
    .map(String::toUpperCase)
    .forEach(System.out::println);</string>

String::toUpperCase is a typical application of类::实例方法. The "referenced method parameters need to be consistent with the second to last parameter of the abstract method, and the first parameter of the instance method is implicit" mentioned in the comment is a brief summary of its mechanism.

Principle analysis

类::实例方法is a method reference expression. When using this form, if the referenced method is an instance method, when called, the first parameter is implicitly passed to the method as this parameter (i.e. the object itself), and the subsequent parameters correspond to the actual parameter of the method.

The Java language specification states:

• If the method reference target is an instance method, the target reference is the first formal parameter of the calling method.
• If the method reference target is an instance method, the parameters of the method calling expression are the second and subsequent formal parameters of the calling method.

This explains how parameters are mapped to the target instance method.

Example details

In the above code, String::toUpperCase refers to the instance method of String class toUpperCase() . toUpperCase() itself has no parameters, but in the map method, it receives String object provided by map method as an implicit this parameter. Therefore, String::toUpperCase correctly converts each String object in the stream to uppercase letters.

Summarize

类::实例方法provides a more concise and expressive way to refer to existing methods, thereby improving the readability and maintainability of the code. Understanding its parameter mapping mechanism is the key to mastering Java method references. By using method references reasonably, you can write more elegant and efficient Java code.

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