1 keyword
2 code blocks
1 Keywords in Java
1.1 static keyword
-
statickeyword :
- Static means, you can modify variables, you can also modify methods, members modified by static, we call static members
-
static features:
- Static members are shared by all objects of the class
- Loaded as the class is loaded, prior to object existence
- It can be called by object or by class name, it is recommended to use class name
-
public class Student { String name; int age; static String school; public void show() { System.out.println(name + "---" + age + "---" + school); } } /* static关键字的特点 : 1 静态成员被该类的所有对象的进行共享 2 静态成员可以通过类名调用 , 也可以通过对象进行调用 , 推荐使用类名 3 静态成员随着类的加载而加载 , 优先于对象存在 */ public class StudentTest { public static void main(String[] args) { Student.school = "XX学院"; Student s = new Student(); s.name = "张三"; s.age = 23; s.show(); Student s2 = new Student(); s2.show(); } } -
Static memory diagram
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Notes on the static keyword
- Only static members can be called in static methods
- Any member can be called in a non-static method
- The this keyword cannot exist in a static method
1.2 final keyword
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final keyword: Translate Chinese to represent the final meaning. It is a keyword in java and also a modifier. It can modify classes, methods, and variables
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Features of final keyword modification
- final modified class: cannot be inherited, no subclass (eunuch class)
- The method modified by fina: cannot be overridden
- final modified variables
- Primitive Data Types: Values cannot be changed
- Reference data type: the address cannot be changed, and the properties of the object can be changed
- Notice :
- Variables modified by final, we call custom constants, naming convention: each letter needs to be capitalized, and multiple words are separated by underscores
- Final modified member variables need to pay attention to the timing of initialization, 1) direct assignment, 2) assignment before the execution of the construction method is completed
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package com.itheima.final_demo; /* final的特点 final修饰的类 , 不能被继承, 也就是没有子类 final修饰的方法 , 不能被重写 final修饰的变量 基本数据类型 : 值不可以发生改变 引用数据类型 : 地址不可发生改变 , 对象的内容可以发生改变 注意 : 1 被final修饰的变量 , 我们叫做自定义常量 , 命名规范 : 每个字母需要大写 , 多个单词之间用下划线分割 2 final修饰成员变量需要注意初始化时机的问题 1) 直接赋值 2) 在构造方法执行完毕前赋值 */ public class FinalDemo1 { public static void main(String[] args) { // final修饰的基本数据类型变量 , 值不能被修改 // final int num = 10; // num = 20; // System.out.println(num); final int[] arr = { 1, 2, 3, 4, 5}; // final修饰的引用数据类型 , 地址不可改发生改变 // arr = new int[3]; // final修饰的引用数据类型 , 对象中的内容可以发生改变 arr[0] = 100; } } // final修饰的类 , 不能被继承, 也就是没有子类 //final class Person { // //} class Person { // final修饰的方法 , 不能被重写 public final void eat() { } } class Student extends Person { // final修饰成员变量需要注意初始化时机的问题 // 要么直接赋值 , 要么在构造方法执行完毕前赋值 // final int num = 10; final int num; public Student() { num = 100; } // @Override // public void eat() { // super.eat(); // } }
1.3 Permission Modifiers in Java
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public – protected – default – private
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package com.bn.permissions_demo1; /* public -- protected -- 默认的 -- private */ public class Fu { public void method1() { } protected void method2() { } void method3() { } private void method4() { } // 同一个包中同一个类 public void show(){ method1(); method2(); method3(); method4(); } } =================================================== package com.bn.permissions_demo1; public class Test { // 同一个包中 , 无关类 public void show(){ Fu f = new Fu(); f.method1(); f.method2(); f.method3(); // f.method4(); } } ==================================================== package com.bn.permissions_demo1; public class Zi extends Fu { // 用一个包中 , 有子类父关系 public void show(){ method1(); method2(); method3(); // method4(); } }package com.bn.permissions_demo2; import com.bn.permissions_demo1.Fu; public class Test { // 不同包中 , 无关类 public void show(){ Fu f = new Fu(); f.method1(); // f.method2(); // f.method3(); // f.method4(); } } ====================================== package com.bn.permissions_demo2; import com.bn.permissions_demo1.Fu; public class Zi extends Fu { // 不同包中 , 有子类父关系 public void show(){ method1(); method2(); // method3(); // method4(); } }
2 code blocks
2.1 Constructing code blocks
- Construction method block: represented by a pair of curly braces, defined outside the method in the class
- Execution timing: Before each construction method is executed, the construction code block will be executed
- Function: Extract the common content of the construction method
package com.bn.code_block;
/*
构造代码块
*/
public class Student {
final int NUM;
{
NUM = 10;
System.out.println("构造代码块...");
}
public Student() {
System.out.println("空参构造...");
}
public Student(int a) {
System.out.println("有参构造...");
}
}
class StudentTest {
public static void main(String[] args) {
Student s = new Student();
Student s2 = new Student(10);
}
}
2.2 Static code block
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Static code block: add the static keyword before a pair of curly braces, defined outside the method in the class
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Execution timing: loaded as the class is loaded, only loaded once
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Function: Generally used to initialize static members
package com.bn.code_block.static_demo; /* 静态代码块 : 1 定义的位置 : 在一对大括号前加上static , 定义在类中方法外 2 执行时机 ; 随着类的加载而执行, 只加载一次 3 可以给类中的静态成员进行初始化数据 */ public class Test { public static void main(String[] args) { new Student(); new Student(10); } }
class Student { static { System.out.println("static code block"); }
public Student() {
System.out.println("空参构造");
}
public Student(int a) {
System.out.println("有参构造");
}
}
### 2.3 局部代码块
- 成员代码块 : 用一对大括号表 , 可以定义在任何的局部位置 , 方法中居多
- 执行时机 : 正常执行(从上往下依次执行)
- 控制变量的局部变量的声明周期
```java
package com.bn.code_block.local_demo;
/*
局部代码块 :
1 位置 : 可以定义任何的局部的位置 , 方法中居多
2 执行时机 : 正常执行(从上往下依次执行)
3 作用 : 控制变量的生命周期 , 变量在使用完毕, 及时释放内存
*/
public class Test {
public static void main(String[] args) {
int num1 = 10;
System.out.println(num1);
// 局部代码块
{
int num2 = 20;
System.out.println(num1);
System.out.println(num2);
}
System.out.println(num1);
// System.out.println(num2);// 作用域不够 , 报错
}
}