commit aa2981f4d3a26ad8fe72c3895984457385597931 Author: icecat <1193498534@qq.com> Date: Fri Mar 21 22:56:25 2025 +0800 first save diff --git a/Java.md b/Java.md new file mode 100644 index 0000000..8c3a11c --- /dev/null +++ b/Java.md @@ -0,0 +1,337 @@ +# Java学习 + +## 静态 static修饰成员的特点 + +![image-20250227144846893](https://gitee.com/icecat2233/picture/raw/master/20250321161353509.png) + +## 运算符 + +### 扩展赋值运算符 + +扩展赋值运算符，内部自带强转效果 + +![image-20250227150015125](https://gitee.com/icecat2233/picture/raw/master/20250321161402334.png) + +### 关系运算符 + +![image-20250227150114816](https://gitee.com/icecat2233/picture/raw/master/20250321161407355.png) + +![image-20250227150145972](https://gitee.com/icecat2233/picture/raw/master/20250321161413451.png) + +![image-20250227151132676](https://gitee.com/icecat2233/picture/raw/master/20250321161419490.png) + +不用记，若想让哪个先运算，只需加小括号 + +## 成员变量和局部变量的区别 + +![image-20250303150638312](https://gitee.com/icecat2233/picture/raw/master/20250321161432068.png) + +## this关键字 + +image-20250303152002372 + +## 标准Javabean + +![image-20250303161759800](https://gitee.com/icecat2233/picture/raw/master/20250321161447983.png) + +## String类的特点 + +![image-20250306160712424](https://gitee.com/icecat2233/picture/raw/master/20250321161455651.png) + +### String类截取方法 + +![image-20250307151219381](https://gitee.com/icecat2233/picture/raw/master/20250321161500034.png) + +![image-20250307153940677](https://gitee.com/icecat2233/picture/raw/master/20250321161506006.png) + +## StringBuilder的特点 + +### 构造方法和介绍 + +![image-20250307163606184](https://gitee.com/icecat2233/picture/raw/master/20250321161512641.png) + +### 提高了字符串的效率 + +![image-20250307154712819](https://gitee.com/icecat2233/picture/raw/master/20250321161519952.png) + +### 常用的成员方法 + +![image-20250307163441799](https://gitee.com/icecat2233/picture/raw/master/20250321161529510.png) + + + +### StringBuilder和StringBuffer区别 + +成员方法一致 + +但是StringBuilder多线程不安全但效率偏高 + +StringBuffer多线程安全但效率偏低 + +## ArrayList集合 + +![image-20250311170523666](https://gitee.com/icecat2233/picture/raw/master/20250321161549694.png) + +![image-20250311170433866](https://gitee.com/icecat2233/picture/raw/master/20250321161545631.png) + +### ArrayList集合成员方法 + +![image-20250312180718819](https://gitee.com/icecat2233/picture/raw/master/20250321161718414.png) + +## extends继承 + +![image-20250317154909941](https://gitee.com/icecat2233/picture/raw/master/20250321161713079.png) + +注意事项： + +image-20250317154930258 + +### 只支持单继承，不支持多继承，支持多层继承 + +例子： + +image-20250317161000220 + +不能多继承原因：如果多继承 会导致子类若执行俩父类中声明完全相同方法时，不知道会执行哪一个 + +## 权限修饰符 + +![image-20250317160532814](https://gitee.com/icecat2233/picture/raw/master/20250321161656034.png) + +***其中protected***不常用访问麻烦 需要在子类中通过创建新方法然后通过super方法进行调用 + +## This和super + +### 开闭原则： + +![image-20250318161204589](https://gitee.com/icecat2233/picture/raw/master/20250321161649403.png) + +例如 + +1.0版本有三个成员变量 + +1.1版本有四个成员变量 + +则在不修改原来代码的基础上，对原有构造方法进行重载，加上新成员变量 + +代码如下 + +```java +public A(){ + +this.(a,b,c) + +this.d = d + +} +``` + +### this和super + +![image-20250318161614034](https://gitee.com/icecat2233/picture/raw/master/20250321161643294.png) + +## final关键字 + +![image-20250318163415671](https://gitee.com/icecat2233/picture/raw/master/20250321161635539.png) + +### 细节补充： + +![image-20250318163435173](https://gitee.com/icecat2233/picture/raw/master/20250321161630257.png) + +## abstract抽象类和抽象方法 + +![image-20250318170528242](https://gitee.com/icecat2233/picture/raw/master/20250321161624677.png) + +### 什么时候用： + +把子类共性的方法抽取到父类，发现描述不清时，就可以把此方法变为抽象方法来让子类重写 + +### 定义格式 + +![image-20250318170848511](https://gitee.com/icecat2233/picture/raw/master/20250321161619634.png) + +### abstract关键字冲突 + +final：被final修饰的方法无法进行重写 + +private：抽象方法需子类重写。private修饰后不能跨类调用无法重写 + +static：类名可调用static修饰的方法，对于抽象方法无意义 + +### abstract抽象类注意事项 + +1.不能实例化（不能创建对象） + +防止调用内部方法 + +2.存在构造方法 + +通过子类super调用抽象父类构造方法 + +3.存在普通方法体 + +可以通过子类进行访问 + +4.抽象类的子类 + +要么重写所有抽象方法，要么本身就是抽象类 + +## 接口介绍： + +接口：体现的思想是对规则的声明 + +**成员变量：默认被public static final所修饰，所以只能为常量** + +**成员方法：默认被public abstract所修饰，只能为抽象方法*****但是JDK7 和8有一些新特性*** + +接口无构造方法 + +​ Java中更多的体现是对行为的抽象 + +image-20250319161530044 + +### 定义方法： + +```java +interface inter{ + public abstract void a(); + public abstract void b(); +} + +class interfac implements inter{ + +​ @Override +​ public void a() { + +​ } + +​ @Override +​ public void b() { + +​ } +} +``` + +没啥用 做个测试 哈哈哈哈哈 图床使用 + +![image-20250319174939799](https://gitee.com/icecat2233/picture/raw/master/20250319174950083.png) + +## 多态： + +### 前提： + +1. 有继承/实现关系 +2. 有方法重写 +3. **父类引用指向子类对象** + +​ + +### 成员访问特点： + +成员变量：编译看左边（父类）运行看左边（父类） + +成员方法：编译看左边（父类）运行看右边（子类） + +- 在编译时会检查父类中有无这个方法。无就报错，有则编译通过，但执行子类方法 + +- 原因：防止父类方法为抽象方法 + +- 多态创建对象，调用静态成员 + + 静态成员仅推荐用类名调用，即使通过对象名调用也是假象，生成字节码文件后会自动改为类名调用 + +### 好处和弊端： + +拓展性！拓展性！我理解为复用性也有 代码量减少了一点 + +image-20250320151226123 + +#### 多态转型：转型问题 + +![image-20250320153129346](https://gitee.com/icecat2233/picture/raw/master/20250320153130897.png) + +![image-20250320153229495](imgh/image-20250320153229495.png) + +解决方法： + +instanceof类型 + +格式：`对象名 instanceof 类型` + +判断一个对象是否是右边的引用类型 + +返回boolean类型的结果 + +## JDK8和9版本的新特性： + +### JDK8中： + +允许在接口定义非抽象方法，但是需要关键字default修饰，这些方法时默认方法 + +作用：解决接口升级问题 + +接口中默认方法定义格式 + +格式：`public default 返回值类型 方法名(参数列表){}` + +重新interface接口里的默认方法时，要将标识interface接口名加在前 + +例如 `inter.super.method()` + +#### 2:接口中允许定义static静态方法 + +接口中静态方法的定义格式： + +格式：public static 返回值类型 方法名(参数列表){} + +注意事项： + +![image-20250321162816640](https://gitee.com/icecat2233/picture/raw/master/20250321162818542.png) + +### JDK9中： + +![image-20250321162851893](imgh/image-20250321162851893.png) + +#### 为和允许定义私有方法： + +![image-20250321162940062](https://gitee.com/icecat2233/picture/raw/master/20250321162941500.png) + +提升复用性，减少冗余代码 + +## 代码块 + +**了解即可** + +![image-20250321171212459](https://gitee.com/icecat2233/picture/raw/master/20250321171221621.png) + +## 内部类： + +### 成员内部类：了解 + +**了解即可** + +翻看Java源代码会有内部类，所以得了解，无知识盲区 + +![image-20250321172536776](https://gitee.com/icecat2233/picture/raw/master/20250321172539305.png) + +#### 内部类成员访问： + +内部类中，访问外部类成员：直接访问，私有也可以 + +外部类中，访问内部类成员：需要创建对象访问， + +![image-20250321172717338](https://gitee.com/icecat2233/picture/raw/master/20250321172718684.png) + +#### 为何要学习内部类？ + +**封装性更好** + +例如： + +![image-20250321172953230](https://gitee.com/icecat2233/picture/raw/master/20250321172955117.png) + +### 静态内部类：了解 + +### 局部内部类：了解 + +### 匿名内部类：必须掌握！！ diff --git a/imgh/image-20250227144846893.png b/imgh/image-20250227144846893.png new file mode 100644 index 0000000..2887bbb Binary files /dev/null and b/imgh/image-20250227144846893.png differ diff --git a/imgh/image-20250227150015125.png b/imgh/image-20250227150015125.png new file mode 100644 index 0000000..d9928b9 Binary files /dev/null and b/imgh/image-20250227150015125.png differ diff --git a/imgh/image-20250227150114816.png b/imgh/image-20250227150114816.png new file mode 100644 index 0000000..d5e48fc Binary files /dev/null and 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