JAVA中主数据类型的过载问题

在JAVA中，主数据类型能从一个“较小”的类型自动转换为一个“较大“的类型，然而也仅次于较大，并不能自动越级转化，比如说主数据类型有（从“较小“到“较大”顺序排列）：char、byte、short、int、long、float、double。int类型能够在未强制转换时自动转换为long而不会转换为其它更高一级的float、double等类型。但是在这些主数据类型当中，char类型略有不同，char类型在没有发现一个准确的char匹配时，会自动转换为int类型。接下来给出一个例子，此程序运用逐步减少某一函数来达到程序的自动转换：

import java.util.*;public class Project6 {static void prt(String s){System.out.println(s);}void f1(char x){prt("f1(char)");}void f1(byte x){prt("f1(byte)");}void f1(short x){prt("f1(short)");}void f1(int x){prt("f1(int)");}void f1(long x){prt("f1(long)");}void f1(float x){prt("f1(float)");}void f1(double x){prt("f1(double)");}void f2(byte x){prt("f2(byte)");}void f2(short x){prt("f2(short)");}void f2(int x){prt("f2(int)");}void f2(long x){prt("f2(long)");}void f2(float x){prt("f2(float)");}void f2(double x){prt("f2(double)");}void f3(short x){prt("f3(short)");}void f3(int x){prt("f3(int)");}void f3(long x){prt("f3(long)");}void f3(float x){prt("f3(float)");}void f3(double x){prt("f3(double)");}void f4(int x){prt("f4(int)");}void f4(long x){prt("f4(long)");}void f4(float x){prt("f4(float)");}void f4(double x){prt("f4(double)");}void f5(long x){prt("f5(long)");}void f5(float x){prt("f5(float)");}void f5(double x){prt("f5(double)");}void f6(float x){prt("f6(float)");}void f6(double x){prt("f6(double)");}void f7(double x){prt("f7(double)");}void testConstVal(){prt("Testing with 5:");f1(5);f2(5);f3(5);f4(5);f5(5);f6(5);f7(5);}void testChar(){char x = 'x';prt("char argument:");f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);}void testByte(){byte x = 0;prt("byte argument:");f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);}void testShort(){short x = 0;prt("short argument:");f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);}void testInt(){int x = 0;prt("int argument:");f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);}void testLong(){long x = 0;prt("long argument:");f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);}void testFloat(){float x = 0;prt("float argument:");f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);}void testDouble(){double x = 0;prt("double argument:");f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);}public static void main(String[] args) {// TODO Auto-generated method stubProject6 p = new Project6();p.testConstVal();p.testChar();p.testByte();p.testShort();p.testInt();p.testLong();p.testFloat();p.testDouble();}}

此程序的运行结果为：

Testing with 5:
f1(int)
f2(int)
f3(int)
f4(int)
f5(long)
f6(float)
f7(double)
char argument:
f1(char)
f2(int)
f3(int)
f4(int)
f5(long)
f6(float)
f7(double)
byte argument:
f1(byte)
f2(byte)
f3(short)
f4(int)
f5(long)
f6(float)
f7(double)
short argument:
f1(short)
f2(short)
f3(short)
f4(int)
f5(long)
f6(float)
f7(double)
int argument:
f1(int)
f2(int)
f3(int)
f4(int)
f5(long)
f6(float)
f7(double)
long argument:
f1(long)
f2(long)
f3(long)
f4(long)
f5(long)
f6(float)
f7(double)
float argument:
f1(float)
f2(float)
f3(float)
f4(float)
f5(float)
f6(float)
f7(double)
double argument:
f1(double)
f2(double)
f3(double)
f4(double)
f5(double)
f6(double)
f7(double)
可以看到在char类型转化过程中的一些不同之处。那么如果我们将转化过程逆转，由“较大”的数据类型转化到“较小”的数据类型时会发生什么？修改上述代码，只对double类型进行测试：

import java.util.*;public class Project7 {static void prt(String s){System.out.println(s);}void f1(char x){prt("f1(char)");}void f1(byte x){prt("f1(byte)");}void f1(short x){prt("f1(short)");}void f1(int x){prt("f1(int)");}void f1(long x){prt("f1(long)");}void f1(float x){prt("f1(float)");}void f1(double x){prt("f1(double)");}void f2(char x){prt("f2(char)");}void f2(byte x){prt("f2(byte)");}void f2(short x){prt("f2(short)");}void f2(int x){prt("f2(int)");}void f2(long x){prt("f2(long)");}void f2(float x){prt("f2(float)");}void f3(char x){prt("f3(char)");}void f3(byte x){prt("f3(byte)");}void f3(short x){prt("f3(short)");}void f3(int x){prt("f3(int)");}void f3(long x){prt("f3(long)");}void f4(char x){prt("f4(char)");}void f4(byte x){prt("f4(byte)");}void f4(short x){prt("f4(short)");}void f4(int x){prt("f4(int)");}void f5(char x){prt("f5(char)");}void f5(byte x){prt("f5(byte)");}void f5(short x){prt("f5(short)");}void f6(char x){prt("f6(char)");}void f6(byte x){prt("f6(byte)");}void f7(char x){prt("f7(char)");}void testDouble(){double x = 0;prt("double argument:");f1(x);f2((float)x);f3((long)x);f4((int)x);f5((short)x);f6((byte)x);f7((char)x);}public static void main(String[] args) {// TODO Auto-generated method stubProject7 p = new Project7();p.testDouble();}}

程序的运行结果如下：

可以看到，这是一种缩小转换，只有在程序中进行强制类型转换时，“较大”的主数据类型才可以转为“较小”的类型，如果不加以强制转换标志编译器会报错，编译器强迫我们明确转换的类型，在转换的过程中精度一定会丧失。