Java中内部类和静态嵌套类的主要区别是什么?设计/实现是否在选择其中一个方面发挥作用?


当前回答

嵌套类的另一个用例,除了已经提到的那些用例之外,是当嵌套类具有只能从外部类访问的方法时。这是可能的,因为外部类可以访问嵌套类的私有构造函数、字段和方法。

在下面的示例中,银行可以发行具有私有构造函数的Bank.CreditCard,并可以使用Bank.credit card的私有setLimit(…)实例方法根据当前银行策略更改信用卡的限额。从任何其他类只能访问Bank.CreditCard的公共方法。

public class Bank {

    // maximum limit as per current bank policy
    // is subject to change
    private int maxLimit = 7000;

    // ------- PUBLIC METHODS ---------

    public CreditCard issueCard(
            final String firstName,
            final String lastName
    ) {
        final String number = this.generateNumber();
        final int expiryDate = this.generateExpiryDate();
        final int CVV = this.generateCVV();
        return new CreditCard(firstName, lastName, number, expiryDate, CVV);
    }


    public boolean setLimit(
            final CreditCard creditCard,
            final int limit
    ) {
        if (limit <= this.maxLimit) {    // check against current bank policy limit
            creditCard.setLimit(limit);  // access private method Bank.CreditCard.setLimit(int)
            return true;
        }
        return false;
    }

    // ------- PRIVATE METHODS ---------

    private String generateNumber() {
        return "1234-5678-9101-1123";   // the numbers should be unique for each card
    }


    private int generateExpiryDate() {
        return 202405;                  // date is YYYY=2024, MM=05
    }


    private int generateCVV() {
        return 123;                     // is in real-life less predictable
    }


    // ------- PUBLIC STATIC NESTED CLASS ---------

    public static final class CreditCard {
        private final String firstName;
        private final String lastName;
        private final String number;
        private final int expiryDate;
        private final int CVV;

        private int balance;
        private int limit = 100; // default limit

        // the constructor is final but is accessible from outer class
        private CreditCard(
                final String firstName,
                final String lastName,
                final String number,
                final int expiryDate,
                final int CVV
        ) {
            this.firstName = firstName;
            this.lastName = lastName;
            this.number = number;
            this.expiryDate = expiryDate;
            this.CVV = CVV;
        }

        // ------- PUBLIC METHODS ---------

        public String getFirstName() {
            return this.firstName;
        }

        public String getLastName() {
            return this.lastName;
        }

        public String getNumber() {
            return this.number;
        }

        public int getExpiryDate() {
            return this.expiryDate;
        }

        // returns true if financial transaction is successful
        // otherwise false
        public boolean charge(final int amount) {
            final int newBalance = this.balance - amount;
            if (newBalance < -this.limit) {
                return false;
            }
            this.balance = newBalance;
            return true;
        }

        // ------- PRIVATE METHODS ---------

        private int getCVV() {
            return this.CVV;
        }

        private int getBalance() {
            return this.balance;
        }

        private void setBalance(final int balance) {
            this.balance = balance;
        }

        private int getLimit() {
            return limit;
        }

        private void setLimit(final int limit) {
            this.limit = limit;
        }
    }
}

其他回答

我已经说明了java代码中可能出现的各种可能的正确和错误场景。

    class Outter1 {

        String OutStr;

        Outter1(String str) {
            OutStr = str;
        }

        public void NonStaticMethod(String st)  {

            String temp1 = "ashish";
            final String  tempFinal1 = "ashish"; 

            //  below static attribute not permitted
            // static String tempStatic1 = "static";    

            //  below static with final attribute not permitted         
            // static final String  tempStatic1 = "ashish";  

            // synchronized keyword is not permitted below          
            class localInnerNonStatic1 {            

                synchronized    public void innerMethod(String str11) {
                    str11 = temp1 +" sharma";
                    System.out.println("innerMethod ===> "+str11);
                }

                /* 
        //  static method with final not permitted
          public static void innerStaticMethod(String str11) { 

                    str11 = temp1 +" india";
                    System.out.println("innerMethod ===> "+str11);
                }*/
            }

            // static class not permitted below
            //  static class localInnerStatic1 {   }                            

        }

        public static  void StaticMethod(String st)     {

            String temp1 = "ashish";
            final String  tempFinal1 = "ashish"; 

            // static attribute not permitted below
            //static String tempStatic1 = "static";     

            //  static with final attribute not permitted below
            // static final String  tempStatic1 = "ashish";                         

            class localInnerNonStatic1 {
                public void innerMethod(String str11) {
                    str11 = temp1 +" sharma";
                    System.out.println("innerMethod ===> "+str11);
                }

                /*
    // static method with final not permitted
    public static void innerStaticMethod(String str11) {  
                    str11 = temp1 +" india";
                    System.out.println("innerMethod ===> "+str11);
                }*/
            }

            // static class not permitted below
            //  static class localInnerStatic1 {   }    

        }

        // synchronized keyword is not permitted
        static  class inner1 {          

            static String  temp1 = "ashish";
            String  tempNonStatic = "ashish";
            // class localInner1 {

            public void innerMethod(String str11) {
                str11 = temp1 +" sharma";
                str11 = str11+ tempNonStatic +" sharma";
                System.out.println("innerMethod ===> "+str11);
            }

            public static void innerStaticMethod(String str11) {
                //  error in below step
                str11 = temp1 +" india";    
                //str11 = str11+ tempNonStatic +" sharma";
                System.out.println("innerMethod ===> "+str11);
            }
            //}
        }

        //synchronized keyword is not permitted below
        class innerNonStatic1 {             

//This is important we have to keep final with static modifier in non
// static innerclass below
            static final String  temp1 = "ashish";  
            String  tempNonStatic = "ashish";
            // class localInner1 {

            synchronized    public void innerMethod(String str11) {
                tempNonStatic = tempNonStatic +" ...";
                str11 = temp1 +" sharma";
                str11 = str11+ tempNonStatic +" sharma";
                System.out.println("innerMethod ===> "+str11);
            }

            /*
            //  error in below step
            public static void innerStaticMethod(String str11) {   
                            //  error in below step
                            // str11 = tempNonStatic +" india";                     
                            str11 = temp1 +" india";
                            System.out.println("innerMethod ===> "+str11);
                        }*/
                    //}
                }
    }

针对Java和/或嵌套类新手的学习者

嵌套类可以是:1.静态嵌套类。2.非静态嵌套类。(也称为内部类)=>请记住这一点

1.内部类例子:

class OuterClass  {
/*  some code here...*/
     class InnerClass  {  }
/*  some code here...*/
}

内部类是嵌套类的子集:

内部类是特定类型的嵌套类内部类是嵌套类的子集可以说内部类也是嵌套类,但不能说嵌套类也是内部类。

内层专业:

内部类的实例可以访问外部类的所有成员,即使是标记为“private”的成员

2.静态嵌套类:例子:

class EnclosingClass {
  static class Nested {
    void someMethod() { System.out.println("hello SO"); }
  }
}

案例1:从非封闭类实例化静态嵌套类

class NonEnclosingClass {

  public static void main(String[] args) {
    /*instantiate the Nested class that is a static
      member of the EnclosingClass class:
    */

    EnclosingClass.Nested n = new EnclosingClass.Nested(); 
    n.someMethod();  //prints out "hello"
  }
}

案例2:从封闭类实例化静态嵌套类

class EnclosingClass {

  static class Nested {
    void anotherMethod() { System.out.println("hi again"); } 
  }

  public static void main(String[] args) {
    //access enclosed class:

    Nested n = new Nested(); 
    n.anotherMethod();  //prints out "hi again"
  }

}

静态类专业:

静态内部类只能访问外部类的静态成员,不能访问非静态成员。

结论:问:Java中内部类和静态嵌套类的主要区别是什么?答:只需仔细阅读上面提到的每门课的细节。

嗯……内部类是嵌套类……您是指匿名类和内部类吗?

编辑:如果您实际上是指内部v.s.匿名:内部类只是在类中定义的类,例如:

public class A {
    public class B {
    }
}

…而匿名类是匿名定义的类的扩展,因此没有定义实际的“类”,如:

public class A {
}

A anon = new A() { /* You could change behavior of A here */ };

进一步编辑:

维基百科声称Java存在差异,但我已经用Java工作了八年,这是我第一次听到这样的区别——更不用说那里没有引用来支持这一说法……总之,内部类是在类中定义的类(静态或非静态),嵌套只是另一个意思相同的术语。

静态和非静态嵌套类之间有一个微妙的区别……基本上,非静态内部类可以隐式访问封闭类的实例字段和方法(因此它们不能在静态上下文中构造,这将是编译器错误)。另一方面,静态嵌套类不具有对实例字段和方法的隐式访问,并且可以在静态上下文中构造。

这些术语可以互换使用。如果你真的想变得迂腐,那么你可以定义“嵌套类”来引用一个静态的内部类,这个类没有封闭的实例。在代码中,您可能有如下内容:

public class Outer {
    public class Inner {}

    public static class Nested {}
}

但这并不是一个被广泛接受的定义。

首先,没有这样的类称为静态类。与内部类一起使用的静态修饰符(称为嵌套类)表示它是外部类的静态成员,这意味着我们可以像其他静态成员一样访问它,而不需要外部类的任何实例。(这本来就是静态的好处。)

使用嵌套类和常规内部类之间的区别是:

OuterClass.InnerClass inner = new OuterClass().new InnerClass();

首先我们可以实例化Outerclass,然后我们可以访问Inner。

但如果类是嵌套的,则语法为:

OuterClass.InnerClass inner = new OuterClass.InnerClass();

它使用静态语法作为静态关键字的正常实现。