java协变返回类型

协变返回类型 (Covariant return type)

The covariant return type is that return type which may vary in parent and child class (or subclass).

协变量返回类型是该返回类型，在父类和子类(或子类)中可能有所不同。

Before JDK5 java does not support covariant return type it means (parent return type and child return must be same).

在JDK5 Java不支持协变返回类型之前，这意味着(父返回类型和子返回必须相同)。

In JDK5 onwards java supports covariant return type it means (parent return type and child return can vary if we define).

从JDK5开始，Java支持协变返回类型 (如果我们定义，则父返回类型和子返回可以不同)。

The covariant return type is useful in method overriding when we override a method in the derived class (or subclass) then return type may vary depending on JDK (Before JDK5 return type should be same and after or JDK5 we can consider return type can vary).

当我们重写派生类(或子类)中的方法时， 协变量返回类型在方法重写中很有用，然后返回类型可能会因JDK而异(在JDK5返回类型应该相同之前，或者在JDK5之后，我们可以考虑返回类型可以变化) 。

Example -1

例子-1

class ParentClass1{int a=10,b=20;
public int sum(){return a+b;
}
}
class ChildClass1 extends ParentClass1{int c=30, d=40;
public int sum(){return (c+d);
}
public static void main(String[] args){ChildClass1 cc1 = new ChildClass1();
ParentClass1 pc1 = new ParentClass1();
int e = cc1.sum();
int f = pc1.sum();
System.out.println("Child class Sum is :"+e);
System.out.println("Parent class Sum is :"+f);
}
}

Output

输出量

D:\Java Articles>java ChildClass1
Child class Sum is :70
Parent class Sum is :30

Example -2

示例-2

class ParentA {}
class ChildB extends ParentA {}
class Base
{ParentA demoA()
{System.out.println("Base class demo");
return new ParentA();
}
}
class Subclass extends Base
{ChildB demoB()
{System.out.println("Subclass demoB");
return new ChildB();
}
}
class MainClass
{public static void main(String args[])
{Base b = new Base();
b.demoA();
Subclass sub = new Subclass();
sub.demoB();
}
}

Output

输出量

D:\Java Articles>java MainClass
Base class demo
Subclass demoB

翻译自: https://www.includehelp.com/java/covariant-return-type.aspx

java协变返回类型