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

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

In recent years, the development of the Java language has been committed to providing a simpler and more efficient programming method. Among them, Java 10 brings an exciting feature to developers-local variable type inference (Local Variable Type Inference). This feature allows us to omit the type when declaring a variable and have the compiler automatically infer the variable's type. This article will focus on how to use the final var keyword in method return values ​​to take advantage of the local variable type inference feature.

Before Java 10, we had to explicitly specify the variable type when declaring a variable. For example, to return the length of a string in a method, we usually write it like this:

public int getStringLength(String str) {
    return str.length();
}

In the above code, we use the int type to declare the return value type of the method. However, sometimes we feel that writing this type is redundant, because we already know that the return type of the str.length() method is always int. In this case, we can take advantage of Java 10's local variable type inference feature and omit the type of the return value.

public var getStringLength(String str) {
    return str.length();
}

In the above code, we use final var to declare the return value type of the method. Var is a new keyword in Java 10, which represents the use of local variable type inference. Since the return type of str.length() is int, var will automatically infer that the return value type is also int.

In actual development, using final var is not just about omitting type declarations. It also makes the code more readable and maintainable. Here is a more complex example:

public var getPersonInfo() {
    final var name = "John Doe";
    final var age = 30;
    final var address = new Address("123 Main St", "City");

    return new Person(name, age, address);
}

In the above code, we use final var to declare three local variables: name, age and address. The types of these variables are String, int and Address respectively. By using var, we can express the meaning of these variables more clearly without having to write an explicit type declaration before each variable. This not only saves the length of the code, but also makes the code more readable.

It should be noted that when using local variable type inference, the variable must be initialized. In other words, we cannot use final var like the following:

public var getPersonInfo() {
    final var name;
    name = "John Doe";
    ...
}

The above code will not compile because name has not been initialized. When using local variable type inference, the compiler needs to be able to infer the type of the variable, and the initialization of the variable is part of the inference process.

In addition, using final var to declare the return value type of a method is not mandatory. If we clearly know the type of the return value and want to pass this information to other developers, we can still write explicit type declarations.

To sum up, the local variable type inference function in Java 10 brings us a simpler and more flexible coding method. By using the final var keyword, we are able to omit type declarations, making the code clearer and more readable. Of course, this doesn't mean we should abuse this feature. In actual development, we should use local variable type inference reasonably to weigh the simplicity and readability of the code.

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