In addition to class in Java, do you know the keyword that defines the class?

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In the past, we used to define classes with class key word , But from Java 16 Start , We will add one more keyword record, It can also be used to define classes .

This record Introduction of keywords , Mainly to provide a more concise 、 Compact final How classes are defined . Let's learn more about record Class details .

Statement record class

Statement record The basic syntax of the class ：

record range(int start, int end){}

We know class The life class can be in a separate file , You can also declare... In other classes . that record Class is the same , It has these kinds of declarations ：

1. A separate document states ：

public record range(int start, int end){}

1. Declare... Inside a class ：

public class DidispaceTest {
    public record range(int start, int end){}
}

1. Intra function declaration ：

public class DidispaceTest {
  public void test() {
    public record range(int start, int end){}
  }
}

record Class explanation

After knowing how to declare , You'll want to use record There are just a few elements declared , What capabilities will it have ？

because record Keyword declaration class is mainly used to simplify the declaration of some classes , So its essence is a special kind of class, Or a template class.

record Declared class , With these characteristics ：

1. It's a final class

2. automatically equals、hashCode、toString function

3. The member variables are public attribute

therefore , For what I wrote before range class , It is equivalent to such a class ：

public final class range{
    final int start;
    final int end;

    public range(int start, int end) {
        this.start = start;
        this.end = end;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        range range = (range) o;
        return start == range.start && end == range.end;
    }

    @Override
    public int hashCode() {
        return Objects.hash(start, end);
    }

    @Override
    public String toString() {
        return "range{" +
                "start=" + x +
                ", end=" + y +
                '}';
    }

    public int start(){
        return start;
    }

    public int end(){
        return end;
    }
}

Because some functions are hidden , stay range When defining , We can't see , So we can try to write such a test code to verify ：

@Test
public void test() {
  range r = new range(100, 200);
  log.info(r.toString());

  range r2 = new range(100, 200);
  log.info(String.valueOf(r.equals(r2)));
}

Define member functions

because record The essence of declaration is also class , Then it is certainly possible to define member functions .

such as , We can do this in record Class to define member functions ：

record range(int start, int end){
  int distance(){
    return end - start;
  }
}

then , You can call it like this ：

range r = new range(100, 200);
r.distance();

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Recommended reading

• You know, Java Hidden classes in ？

• Java 16 Enhanced switch Pattern matching ability

• switch Expression enhancement , Finally can not write break 了
  

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