java小心机(2)| 重载和构造器的小细节

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java小心机,为你诉说java鲜为人知的一面

涉及基本数据类型的重载

基本类型能从一个”较小”的类型自动提升到”较大”的类型（扩展转换）,此过程一旦牵涉到重载,可能就会造成一些混淆

看下以下例子,你是否清楚地知道答案?

import static java.lang.System.*;

public class PrimitiveOverLoading {

    public static void f1(char x){System.out.print("f1(char) ");};

    public static void f1(byte x){System.out.print("f1(byte) ");};

    public static void f1(short x){System.out.print("f1(short) ");};

    public static void f1(int x){System.out.print("f1(int) ");};

    public static void f1(long x){System.out.print("f1(long) ");};

    public static void f1(float x){System.out.print("f1(float) ");};

    public static void f1(double x){System.out.print("f1(double) ");};

    public static void f2(byte x){System.out.print("f2(byte) ");};

    public static void f2(short x){System.out.print("f2(short) ");};

    public static void f2(int x){System.out.print("f2(int) ");};

    public static void f2(long x){System.out.print("f2(long) ");};

    public static void f2(float x){System.out.print("f2(float) ");};

    public static void f2(double x){System.out.print("f2(double) ");};

    public static void f3(short x){System.out.print("f3(short) ");};

    public static void f3(int x){System.out.print("f3(int) ");};

    public static void f3(long x){System.out.print("f3(long) ");};

    public static void f3(float x){System.out.print("f3(float) ");};

    public static void f3(double x){System.out.print("f3(double) ");};

    public static void f4(int x){System.out.print("f4(int) ");};

    public static void f4(long x){System.out.print("f4(long) ");};

    public static void f4(float x){System.out.print("f4(float) ");};

    public static void f4(double x){System.out.print("f4(double) ");};

    public static void f5(long x){System.out.print("f5(long) ");};

    public static void f5(float x){System.out.print("f5(float) ");};

    public static void f5(double x){System.out.print("f5(double) ");};

    public static void f6(float x){System.out.print("f6(float) ");};

    public static void f6(double x){System.out.print("f6(double) ");};

    public static void f7(double x){System.out.print("f7(double) ");};

    void testConstVal(){

        out.print("5: ");

        f1(5);f2(5);f3(5);f4(5);f5(5);f6(5);f7(5);out.println();

    }

    void testDecimalVal(){

        out.print("5.2: ");

        f1(5.2);f2(5.2);f3(5.2);f4(5.2);f5(5.2);f6(5.2);f7(5.2);out.println();

    }

    void testChar(){

        char x = 'x';

        out.print("char: ");

        f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);out.println();

    }

    void testByte(){

        byte x = 0;

        out.print("byte: ");

        f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);out.println();

    }

    void testShort(){

        short x = 0;

        out.print("short: ");

        f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);out.println();

    }

    void testInt(){

        int x = 0;

        out.print("int: ");

        f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);out.println();

    }

    void testLong(){

        long x = 0;

        out.print("long: ");

        f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);out.println();

    }

    void testFloat(){

        float x = 0;

        out.print("float: ");

        f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);out.println();

    }

    void testDouble(){

        double x = 0;

        out.print("double: ");

        f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);out.println();

    }

    public static void main(String[] args){

        PrimitiveOverLoading p = new PrimitiveOverLoading();

        p.testConstVal();

        p.testDecimalVal();

        p.testChar();

        p.testByte();

        p.testShort();

        p.testInt();

        p.testLong();

        p.testFloat();

        p.testDouble();

    }

}

输出如下:

5: f1(int) f2(int) f3(int) f4(int) f5(long) f6(float) f7(double)

5.2: f1(double) f2(double) f3(double) f4(double) f5(double) f6(double )f7(double)

char: f1(char) f2(int) f3(int) f4(int) f5(long) f6(float) f7(double)

byte: f1(byte) f2(byte) f3(short) f4(int) f5(long) f6(float) f7(double)

short: f1(short) f2(short) f3(short) f4(int) f5(long) f6(float) f7(double)

int: f1(int) f2(int) f3(int) f4(int) f5(long) f6(float) f7(double)

long: f1(long) f2(long) f3(long) f4(long) f5(long) f6(float) f7(double)

float: f1(float) f2(float) f3(float) f4(float) f5(float) f6(float) f7(double)

double: f1(double) f2(double) f3(double) f4(double) f5(double) f6(double) f7(do

可得出以下结论:

1.常数值5会被当做int值处理,常数值5.2会被当做double值处理

2.当传入的数据类型小于方法中声明的形参类型，实际数据就会被提升至最近的数据类型。

3.特例:对于char,如果无法恰好找到char参数的方法，且int参数方法存在时，char直接提升至int型

思考题：反过来，当传入的数据类型大于方法中声明的参数类型呢？（自行实践，相关阅读：java”小心机”(1)【资源彩蛋!】第4点类型转换）

构造器中调用构造器

一个类中可能写了多个构造器,有时可能想在一个构造器中调用另一个构造器,以避免重复代码。构造器中调用构造器时，有几点细节需要注意，看下面例子：

注://! 为编译会报错的代码

public class Constructor {

    private String str;

    private int i;

    private float f;

    Constructor(String str){

        this.str = str;

    }

    Constructor(int i){

        this.i = i;

    }

    Constructor(String str,int i){

        //1.无法直接调用,需要使用this添加参数列表调用

        //! Constructor(str);

        this(str);

        //2.只能调用一次构造器

        //! this(i);

        this.i = i;

    }

    Constructor(String str,int i,float f){

         this.str = str;

         this.i = i;

         //3.只能在第一行调用构造器,否则编译不通过

         //! this(str);

    }

    //普通方法

    void normalMethod(){

         //4.除构造器外,编译器禁止其他任何方法调用构造器

         //! this(str);

    }

}

总结:

1.无法直接调用构造器,需要使用this添加参数列表调用

2.只能调用一次构造器

3.只能在构造器中第一行调用构造器,否则编译不通过

4.除构造器外,编译器禁止其他任何方法调用构造器

end~thanks!

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