The difference between a, JDK JRE JVM
JDK: when writing Java code, the necessary software environment, all written in Java project code base and foundation class comes from the JDK,
Jre: is the running environment, Java code is running in the jre, jre can exist alone, the JDK includes the jre,
The JVM: virtual machine, the JVM belongs to the jre,
Second, the principle of Java cross-platform
Compile once, run everywhere, for the class file can be run on any operating system and Java by not cross-platform JVM implementation the cross-platform class files,
Three, handwritten code
Public class Hello {
Public stasic void main (String [] args) {
System. Out.println (" Hello!!!!!! ");
}
}