Local variable type inference in Java 10: How to use var keyword in method return value

Local variable type inference in Java 10: How to use the var keyword in method return values

Introduction:
Java is a statically typed language, and variables usually need to be specified explicitly when declaring them. The type of variable. But in Java 10, the concept of local variable type inference was introduced, and the var keyword can be used instead of the declaration of the variable type. This article will discuss the use of the var keyword in method return values, with code examples.

1. Introduction to local variable type inference:
The local variable type inference of Java 10 allows programmers to omit the declaration of the variable type when declaring local variables. This function is implemented by the var keyword. The compiler infers the actual type of the variable based on the type of the right-hand expression. This can simplify the code and reduce the redundancy of template code.

2. Use the var keyword in the method return value:
In Java 10, the var keyword can be used in the declaration of the method return value. The specific usage is as follows:

1. Explicitly declare the method return value type:
   In early versions of Java, declaring the method return value type is required, as follows:

   public String getMessage() {
   return "Hello World";
   }

2. Use the var keyword to infer the method return value type:
   In Java 10, we can use the var keyword to complete the same declaration, the code is as follows:

   public var getMessage() {
   return "Hello World";
   }

   Here , the compiler will infer that the return value type is String based on the value returned by the method.

3. Combined with Lambda expression:
   In the case of using Lambda expression, we can use the var keyword in the method return value type, the code is as follows:

   public var getSum = (int a, int b) -> a + b;

   The compiler will infer the return value type of the getSum method based on the return value type on the right side of the Lambda expression.

It should be noted that although the var keyword simplifies the code, it also introduces some potential problems. For example, the var keyword is not a dynamic type. The compiler just does type inference based on the type of the current expression, and then declares the variable as the corresponding type. In other words, once a type is inferred, it is fixed and cannot be changed. Therefore, we cannot set different types of values ​​inside the method.

Sample code:
The following is a sample code that uses the var keyword in the method return value to demonstrate its use:

public class VarExample {
   public static void main(String[] args) {
      var message = getMessage();
      System.out.println(message);

      var sum = getSum(5, 3);
      System.out.println(sum);
   }

   public static var getMessage() {
      return "Hello World";
   }

   public static var getSum(int a, int b) {
      return a + b;
   }
}

In the above sample code, the getMessage method A string type value is returned, while the getSum method returns the sum of two integers.

Conclusion:
The concept of local variable type inference was introduced in Java 10, and the declaration of the variable type can be omitted using the var keyword. Using the var keyword in method return values ​​can simplify the code and reduce the redundancy of template code.

Although the var keyword brings convenience, it should also be noted that it is not a dynamic type declaration and cannot be changed after the variable type is determined. Therefore, it is recommended to use the var keyword in appropriate scenarios to improve code readability and ease of maintenance.

Reference materials:
[1] https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html
[2] http://openjdk.java. net/projects/amber/LVTIstyle.html

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