我执行HELLO WORLD的时候会报错误: java.lang.UnsupportedClassVersionError: HelloWorld (Unsupported major.minor version 48.0) at java.lang.ClassLoader.defineClass0(Native Method) at java.lang.ClassLoader.defineClass(Unknown Source) at java.security.SecureClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.access$100(Unknown Source) at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClassInternal(Unknown Source) Exception in thread "main"
源码很简单的呀! /Fig.8-1:Time1.java //Time1 class definition import java.text.DecimalFormat;//used for number formatting//This class maintains the time in 24-hour format public class Time1 extends Object{ private int hour;//0-23 private int minute;//0-59 private int second;//0-59//Time1 constructor initializes each instance variable to zero.Ensure that each Time1 //object starts in a consistent state
public Time1() { setTime(0,0,0); } //Set a new time value using universal time.perform validity checks on the data.set //invalid values to zero. public void setTime(int h,int m,int s) { hour=((h>=0&&h<24)?h:0); minute=((m>=0&&m<60)?m:0); second=((s>=0&&s<60)?s:0); }
//Convert to String in universal-time format public String toUniversalString() { DecimalFormat twoDigits=new DecimalFormat("00"); return twoDigits.format(hour) +":"+twoDigits.format(minute) +":"+twoDigits.format(second); } //Convert to String in Standard-time format public String toString() { DecimalFormat twoDigits=new DecimalFormat("00"); return ((hour==12||hour==0)?12:hour%12)+ ":"+twoDigits.format(minute)+":"+twoDigits.format(second)+ (hour<12?"AM":"PM"); } } // Fig.8-1 TimeTest.java //class TimeTest to exercise class Time1 import javax.swing.JOptionPane;public class TimeTest{ public static void main(String args[]) { Time1 t=new Time1();//calls Time1 constructor String output;
output="The initial universal time is:"+ t.toUniversalString()+ "\nThe initial standard time is:"+ t.toString()+ "\nImplicit toString() call:"+t;
t.setTime(13,27,6); output="\n\nUniversal time after setTime is:"+ t.toUniversalString()+ "\nStandard time after setTime is:"+ t.toString();
JOptionPane.showMessageDialog(null,output,"Testing Class Time1", JOptionPane.INFORMATION_MESSAGE);
System.exit(0); } }java.lang.UnsupportedClassVersionError: TimeTest (Unsupported major.minor version 48.0) at java.lang.ClassLoader.defineClass0(Native Method) at java.lang.ClassLoader.defineClass(Unknown Source) at java.security.SecureClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.access$100(Unknown Source) at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClassInternal(Unknown Source) Exception in thread "main"
如果你写的代码没有错,的话会自动在生成.class文件,在源文件一个地方存放.不知道你建立project,是否用java project方式.
java.lang.UnsupportedClassVersionError: HelloWorld (Unsupported major.minor version 48.0)
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Exception in thread "main"
你先建立一个java project,然后新建一个.java文件,写好之后,会自动编译到指定的目录.
也可以手动编译,选中该porject之后,选project-->rebuild project或rebuild all就可以了
/Fig.8-1:Time1.java
//Time1 class definition
import java.text.DecimalFormat;//used for number formatting//This class maintains the time in 24-hour format
public class Time1 extends Object{
private int hour;//0-23
private int minute;//0-59
private int second;//0-59//Time1 constructor initializes each instance variable to zero.Ensure that each Time1
//object starts in a consistent state
public Time1()
{
setTime(0,0,0);
}
//Set a new time value using universal time.perform validity checks on the data.set
//invalid values to zero.
public void setTime(int h,int m,int s)
{
hour=((h>=0&&h<24)?h:0);
minute=((m>=0&&m<60)?m:0);
second=((s>=0&&s<60)?s:0);
}
//Convert to String in universal-time format
public String toUniversalString()
{
DecimalFormat twoDigits=new DecimalFormat("00");
return twoDigits.format(hour)
+":"+twoDigits.format(minute)
+":"+twoDigits.format(second);
}
//Convert to String in Standard-time format
public String toString()
{
DecimalFormat twoDigits=new DecimalFormat("00");
return ((hour==12||hour==0)?12:hour%12)+
":"+twoDigits.format(minute)+":"+twoDigits.format(second)+
(hour<12?"AM":"PM");
}
}
// Fig.8-1 TimeTest.java
//class TimeTest to exercise class Time1
import javax.swing.JOptionPane;public class TimeTest{
public static void main(String args[])
{
Time1 t=new Time1();//calls Time1 constructor
String output;
output="The initial universal time is:"+
t.toUniversalString()+
"\nThe initial standard time is:"+
t.toString()+
"\nImplicit toString() call:"+t;
t.setTime(13,27,6);
output="\n\nUniversal time after setTime is:"+
t.toUniversalString()+
"\nStandard time after setTime is:"+
t.toString();
t.setTime(99,99,99);
output="\n\nAfter attempting invalid settings "+
"\nUniversal time:"+t.toUniversalString()+
"\nStandard time:"+t.toString();
JOptionPane.showMessageDialog(null,output,"Testing Class Time1",
JOptionPane.INFORMATION_MESSAGE);
System.exit(0);
}
}java.lang.UnsupportedClassVersionError: TimeTest (Unsupported major.minor version 48.0)
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Exception in thread "main"
如果没有在porject/properties/library,选择修改或添加varialbe,然后添加或修改JRE_LIB指到合适的jre/bin就可以了。
编译:即时性的,当存盘时!(我不喜欢,按ctrl+z不能继续往前撤销了)
如果与JDK1。4结合
打开:java view ,点击一个项目,右键,properties,然后,选择:java buidl path
点击右边的libraries,将已有的jar包去掉,换成jak1.4的包,就ok了!
但是,每个项目,都要手工的来重新设者这些东东!
”,他和你jre的版本不兼容,换个jre版本试试!