Static variable, also known as class variable
One, the difference
1. Define the location
Local variable: inside the method
Instance variable: inside the class, outside the method
Static variable: inside the class, outside the method
2. Scope
Local variables: valid in methods.
Instance variables: available in the entire class.
Static variables: available in the entire class and can be called directly by the class
3. Default value
Local variable: no default value, need to be assigned before use
Instance variable: have default value
Static variable: have default value
4. Memory location
Local variables: in the stack memory
Instance variables: in the heap memory
Static variables: in the method area
5. Life cycle
Local variables: the same as the life cycle of the method.
Instance variables: take effect when the object of the class is created, and become invalid after garbage collection.
Static variables: exist as the class is loaded, and disappear as the class disappears
Two, summary of the use of variables
public class Test{
int a;//实例变量,有默认值
public void method(int b;/*局部变量*/){
int c;//局部变量，无默认值
System.out.println(c);
//如果打印直接打印变量 c 程序编译会出错，必须进行赋值
System.out.println(b);
//如果打印直接打印变量 b 程序编译不会出错
//虽然 b 没有进行赋值，但是在方法调用时，必然会给方法传递参数，b 也就被赋值了
}
}
In the process of code writing, the scope of the variable should be minimized. The shorter the variable is in the memory, the higher the performance.
public class Test {
static int a;//静态变量
}
class Test1 {
public static void main(String[] args) {
System.out.println(Test.a);//静态变量在其它类中直接用类名.变量名
}
}