https://github.com/hblvsjtu/java_interview_study

之前一直再看类似于Java编程思想、核心技术等书籍，但是理论终归是理论，还需要看看市场上所需要掌握的技术到底是什么。所以后面将会以Java面试作为主线，加强面试能力和算法能力，希望有一个好的结果^_ ^
https://github.com/hblvsjtu/java_interview_study
interview java
Last synced: 12 months ago
JSON representation
之前一直再看类似于Java编程思想、核心技术等书籍，但是理论终归是理论，还需要看看市场上所需要掌握的技术到底是什么。所以后面将会以Java面试作为主线，加强面试能力和算法能力，希望有一个好的结果^_ ^
Host: GitHub
URL: https://github.com/hblvsjtu/java_interview_study
Owner: hblvsjtu
License: mit
Created: 2019-04-16T08:39:22.000Z (about 7 years ago)
Default Branch: master
Last Pushed: 2019-05-01T15:56:11.000Z (about 7 years ago)
Last Synced: 2025-06-24T07:11:28.495Z (12 months ago)
Topics: interview, java
Language: Java
Size: 4.2 MB
Stars: 2
Watchers: 0
Forks: 0
Open Issues: 0
Metadata Files:
- Readme: README.html
- License: LICENSE
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README

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README
Java_Interview_Study



作者：冰红茶



参考书籍：《Java面试宝典》第4版 《剑指Offer》





之前一直再看类似于Java编程思想、核心技术等书籍，但是理论终归是理论，还需要看看市场上所需要掌握的技术到底是什么。所以后面将会以Java面试作为主线，加强面试能力和算法能力，希望有一个好的结果^_ ^

目录





一、Java程序设计基本概念





1.1 JVM





1.2 类型转换、i++和运算符





1.3 异常





二、传递与引用和循环





2.1 传值与引用





2.2 静态变量和私有变量





2.3 输入/输出流和序列化





2.4 典型的递归问题





三、Java内存管理





3.1 典型的递归问题





2.2 clone





2.3 输入/输出流和序列化





四、面向对象





4.1 典型的递归问题





4.2 clone





一、Java程序设计基本概念


1.1 JVM


1) ClassLoader







ClassLoader在JVM运行的时候加载Java核心的API和用户自定义的类；



bootstrap classloader 启动类加载器

classloader 用户定义加载器



ExtClassLoader 用来加载Java拓展API，也就是/lib/ext

AppClassLoader 用来加载用户自定义类













加载流程：当运行一个程序的时候，JVM启动，运行bootstrap classloader，该ClassLoader加载核心API（ExtClassLoader和AppClassLoader）也在此时候被加载，然后调用ExtClassLoader加载拓展API，最后AppClassLoader加载CLASSPATH目录下定义的Class

JVM加载类的三个步骤



装载 找到相应的class，读入JVM

连接 分成三个阶段：验证class是否符合规格；准备，就是为类变量分配内存的同时设置默认初始值；解释，这里的解释是指根据类中的符号引用查找相应的实体，再把符号引用替换成一个直接引用的过程。

初始化 class文件的初始化







类加载的这种关系称为双亲委派模式，需要注意的是他们之间不是继承关系，而是组合关系，在执行类加载的动作时，首先都是交给父类去加载，如果父类无法加载再交给子类去完成，直到调用用户自定义的类加载器去加载，如果全部都无法加载，就会抛出ClassNotFoundException。出处





父类委托 超类被加载后，子类的Classloader没必要再加载一次，如果没有超类（即自己就是超类）时，就使用bootstrap classloader进行加载。原因有二：





这样做可以避免重复加载

考虑到安全的因素。 如果不采用这种方式，用户就可以自己加载String类来动态代替Java的核心API定义的类型，这样会存在很大的安全隐患。这由于采用了父类委托，String已经在启动的时候被加载，所以用户自定义类无法加载一个自定义的ClassLoader



























一些重要的方法





loadClass 











            class loadClass(String name, boolean resolve)









defineClass 将原始字节组成的数组转换成class对象，并且是final的无法被覆盖

findSystemClass 从本地文件中寻找类文件，如果存在，则使用defineClass转换称class对象







            public class findClass(String name) {

                byte[] data = loadClassData(name);

                return defineClass(name, data, 0, data.length);

            }









findClass Java的自定义加载器 loadClass默认实现调用这个新方法，寻找所有Classload的特殊代码。findClass（）用于写类加载逻辑、loadClass（）方法的逻辑里如果父类加载器加载失败则会调用自己的findClass（）方法完成加载，保证了双亲委派规则。如果不想打破双亲委派模型，那么只需要重写findClass方法即可。如果想打破双亲委派模型，那么就重写整个loadClass方法。cqc__c的CSDN



forName(name, initialize, loader)







            Class.forNmae("something", true, CALLCLASS.class.getClassLoader());



2) Unicode







Java中字符只以一种形式存在，那就是Unicode





            char han = '永'；

            System.out.printf("%x", (short)han);

            // 0x6c38







JVM约定字符只分为两个部分，JVM内部和OS文件系统的，在JVM内部统一使用Unicode，当字符被输入输出的时候，就会进行编码转换。也就是说，编码转换只会发生在边界的地方。而I/O也分两大阵型，一个是面向字符，另一个是面向字节





面向类型

特点

使用范围

字节

保证系统中文件的二进制内容和读入JVM内部的二进制内容一致，不进行编码转换

视频或者音频文件 InputStream和OutputStream类为后缀的类

字符

保证系统中文件的字符和读入内存的字符一致，隐式地使用系统默认的编码方式做编码转换（而且这种编码方式无法自己选择）

以Reader和Write为后缀的类

字节-字符

Java中能指定转换编码的地方也就在字符和字节转换的地方

InputStreamReader类和OutputStreamWriter类

1.2 类型转换、i++和运算符


1) 类型转换







数字和字节相加，会将数字进行隐式转换成字符

数字和字节相减，会在编译的时候报错

数字和字符相加，会将字符进行隐式转换成Ascii码再进行相加减

系统默认的浮点类型是double,所以如果是float类型需要在数字后加上f标记进行转换

short后面不需要加S，会执行自动装箱

char后面也不需要加c,char向高级类型转换时会转化成Ascii码

高级类型转低级类型需要强行转换，而且只能是针对数值类型而非包装类进行转换，否则会出错





            package Interview;

            /**

             * @author LvHongbin

             *

             */

            public class Go {

                public static void main(String[] arg) {

                    System.out.println(1 + "0");

                    System.out.println("1" + 0);

                    System.out.println(1 + '0');

                    System.out.println('1' + 0);

                }

            }

            // 控制台

            10

            10

            49

            49



2) 包装类







Character、Boolean、Integer、Long、Float、Double共6个

String和Data本身就是类，所以没有包装类这一说法

过渡型类型转换





            float f = 100.0f;

            Float F = new Float(f);

            double d = F.doubleValue();













3) 中间缓存机制







适用与自增的变量同时出现在等号左右两边





            int j1 = 0 ;

            int j2 = 0 ;

            int j3 = 0 ;

            for(int i =0; i<100; i++) {

                j1 = j1++;

                j2 = ++j2;

            }

            j3 = j3++ + j3++ + j3++ + ++j3;

            System.out.printf("j1 = %d, j2 = %d, j3 = %d", j1, j2, j3);

            // 控制台

            j1 = 0, j2 = 100, j3 = 7

            j1 = j1++;

            等效于

            temp = j1;

            j1 = j1 + 1;

            j1 = temp;

            j2 = ++j2;

            等效于

            j2= j2 + 1;

            temp = j2;

            j2 = temp;

            j3 = 0 + 1 + 2 + 4 = 7



4) 三目运算符







返回类型会进行自动匹配，而匹配的类型跟问号后的值相同





            System.out.println("value = "  + (true == false ? 10.9 : 9));

            // 控制台

            value = 9.0



5) 移位操作符







移位的位数先要进行模32运算





            System.out.println("value = "  + (32 >> 33));

            // 控制台

            value = 16



1.3 异常


1) 异常类型







Throwable类衍生Error类和Exception类



Error类程序无法靠自身解决，所以Java不作处理，所以是unchecked exception

Exception也分为运行时错误（如被0除、空字符串、空句柄）和非运行时错误（如IOException），对于运行时错误，由于编译器无法在静态检查中检查出，所以是unchecked exception







Java编译器要求捕获或者声明所有非运行时错误

针对





2) 异常关键字







try、catch、finally分别有自己独立的代码块，相互之间不能访问，如果需要访问则可以把变量放在关键字代码块之前

throw 用在方法体内部

throws 用在方法体外部，用于声明可能会抛出的异常，里面可以不写catch





3) finally







即便try()代码块内部有return语句，如果存在finally关键字，那么finally里面的方法块会在return前执行





            /**

             * 

             */

            package Interview;

            /**

             * @author LvHongbin

             *

             */

            public class Go {

                public static void main(String[] arg) {

                    System.out.println(fin());

                }

                private static int fin() {

                    try {

                        System.out.println("I am try");

                        return "I am return";

                    }finally {

                        System.out.println("I am finally");

                    }

                }

            }

            // 控制台

            I am try

            I am finally

            I am return





二、传递与引用


2.1 传值与传引用


1) 规律







不管Java参数类型是什么，一律传递的是参数的副本

如果参数是值，那么传得就是值的副本

如果参数是引用，那么传得就是引用的副本，也就是“被复制的钥匙”，这就意味着不能修改“原来的钥匙”

这里跟C++不同，C++在传递引用的时候是真的传递引用，而Java是传递引用的副本。其实两者在方法里都可以改掉被引用的内容。





2.2 静态变量和私有变量


1) 初始化







定义在类中的变量会被赋予一个默认的值





2) 静态方法







静态方法访问外部变量或者方法时只能访问静态变量或者静态方法，否则会出错





2.3 输入/输出流和序列化


1) 读入有很多数据的大文件







new StringBuffer(new InputStreamReader(new FileInputStream(“file.name”)));





2) 从键盘输入数字的两种方法







使用Scanner类





            Scanner scan = new Scanner(System.in);

            System.out.println("请输入第一个数字：");

            int i1 = scan.nextInt();

            System.out.println("请输入第二个数字：");

            int i2 = scan.nextInt();

            System.out.println("两数之和：" + (i1 + i2));

            scan.close();

            // 控制台

            请输入第一个数字：

            1

            请输入第二个数字：

            2

            两数之和：3







使用BufferedReader类和InputStreamReader类





            int i3 = 0;

            int i4 = 0;

            BufferedReader buffer = new BufferedReader(new InputStreamReader(System.in));

            try {

                System.out.println("请输入第一个数字：");

                i3 = Integer.parseInt(buffer.readLine());

                System.out.println("请输入第二个数字：");

                i4 = Integer.parseInt(buffer.readLine());   

            } catch (IOException e) {

                // TODO Auto-generated catch block

                e.printStackTrace();

            }finally {

                System.out.println("两数之和：" + (i3 + i4));

                try {

                    buffer.close();

                } catch (IOException e) {

                    // TODO Auto-generated catch block

                    e.printStackTrace();

                }

            }

            // 控制台

            请输入第一个数字：

            1

            请输入第二个数字：

            2

            两数之和：3







StringBuffer是一个处理字符串的类，而BufferReader是一个I/O流

字符流和字节流分类



























3) 序列化







Java的“对象序列化”能将一个实现了Serializable接口的对象转换成一组byte，这样日后要用的时候就可以把这些byte数据恢复成对象

使用ObjectInputStream和ObjectOutputStream类

调用readObject()方法和writeObject()方法

主要是用于远程传递对象





2.4 典型的递归问题


1) 斐波那契数列



            private static int fib(int i) {

                return i<3 ? 1 : fib(i-1) + fib(i-2);

            }





三、Java内存管理


3.1 垃圾收集


1) Java垃圾收集机制







优势：Java不像C和C++，不要求程序员显式地分配内存，释放内存。避免很多潜在的问题，Java在创建对象时会自动分配内存，并当该对象的引用不存在时释放这块内存

工作机制：Java使用对象表来将软指针映射为对象的引用。之所以称为“软指针”，因为这些指针并不是直接指向对象，而是指向对象的引用。使用软指针，Java的垃圾收集器可以以单独的线程在后台存在，并依次检查每一个对象，通过更改对象表项来标记对象、移除对象、移动对象或者检查对象。

调用System类中的静态gc()方法可以运行垃圾收集器，但不能保证立即回收指定对象，只是向JVM发出回收垃圾的申请而已。具体的原理请参考深入理解Java虚拟机



调用protected void finalize() throws Throwable或者进行重写。在finalize()方法返回之后，对象消失，垃圾收集开始执行。属于一种强制执行垃圾回收的方法。

判断一块内存空间是否符合垃圾收集器收集的标准，只有两条原则



给对象赋予了null，而且再也没有被调用

给对象赋予了新值，即重新分配了内存，那么旧的内存可以被回收







养成良好的编程习惯，让引用变量退出活动域后，自动设置为null，暗示垃圾手机器来收集该对象。注意，局部变量不再使用的时候，没有必要显式设置为null，对于这些变量的引用将随着方法的退出而自动清除。





2) Java中的内存泄露







在Java中，内存泄露就是存在一些被分配的对象，这些对象有两个特点：



对象是可达的，即在有向图中，存在通路可以与其相连

对象是无用的，即程序以后不会再使用这些对象







本质上是占用着内存却不会再被使用的对象













3.2 clone  

















1) 特点







clone()是Object的一个方法，任何类都自动继承拥有这个类

但是继承而来的clone()方法类不能直接调用，需要显式添加“implements Cloneable”作为标记

“implements Cloneable”只是起一个标记的作用，不需要重写或者覆盖clone()方法，这是由Javac或者Java决定的





2) 检验类型







clone()方法对对象是否属于cloneable类型是要进行检验

证据就是类必须实现Cloneable接口，但是这个接口的实现只是作为标记，不起实质上的作用





3) 深拷贝的定义







x.clone() != x

x.clone().getClass() == x.getClass()

x.equal(x.clone()) == true;







四、面向对象


4.1 类和对象


1) equal







貌似只要是不同的对象就不相等

除非你重写equal方法





            Go go1 = new Go();

            Go go2 = new Go();

            Object go3 = new Go();

            System.out.println(go1 == go2);

            System.out.println(go1.equals(go2));

            System.out.println(go1.equals(go1.clone()));

            System.out.println(go2 == go3);

            System.out.println(go3.equals(go2));

            System.out.println(go2.equals(go3));

            // 控制台

            false

            false

            false

            false

            false

            false



2) 创建对象的几种方式







new关键字

clone()方法

反序列化 ObjectInputStream()

运用反射的方法，调用java.lang.Class或者java.lang.reflect.constructor类





3) 嵌套类innner class







静态内部类意味着：



创建一个static内部类的对象，不需要一个外部类对象

不能从一个static内部类对象中访问外部类对象

4.2 集合类
  











1) 分类







Collection 类似于数组



List 以特定的次序储存元素，取出时未必按照原来的顺序



ArrayList 擅长随机访问元素，如果需要在中间插入、移动、删除元素则会花费较多的时间

LinkedList 不擅长随机访问元素，如果需要在中间插入、移动、删除元素则会花费较少的时间

Vector 不进行边界检查，可以确切知道它所持有的对象隶属什么类别，Vector总比ArrayList慢







Set 每个值只保存一个对象，不能还有重复的元素，顺序是随机的



HashSet 使用散列函数

TreeSet 使用红黑树

LinkedHashSet 使用链表结合散列函数







Queue 先进先出



PriorityQueue













Map 类似于键名对 不允许有重复值



HashMap 适用于快速查找 允许一个null键和多个null值

HashTable 

HashTree 用来维护排序状态，比较玄学

TreeMap 适用于已经排好序的的序列











2) 特点







List、Set、Map将所有对象都是为Object

Collection、List、Set、Map都是接口，不能实例化。只有他们的子类才能创建始类

Collections是一个帮助类，它提供一系列静态的方法实现对各种集合的搜索、排序、线程完全化操作，如





            List<Integer> list = new ArrayList<Integer>();

            list.add(1);

            list.add(9);

            list.add(6);

            list.add(3);

            for(int i: list) {

                System.out.print(i);

            }

            Collections.sort(list);

            for(int i: list) {

                System.out.print(i);

            }

            //控制台

            19631369





-
https://github.com/hblvsjtu/java_interview_study

Awesome Lists containing this project

Java_Interview_Study

作者：冰红茶

参考书籍：《Java面试宝典》第4版 《剑指Offer》

目录

一、Java程序设计基本概念

1.1 JVM

1) ClassLoader

2) Unicode

1.2 类型转换、i++和运算符

1) 类型转换

2) 包装类

3) 中间缓存机制

4) 三目运算符

5) 移位操作符

1.3 异常

1) 异常类型

2) 异常关键字

3) finally

二、传递与引用

2.1 传值与传引用

1) 规律

2.2 静态变量和私有变量

1) 初始化

2) 静态方法

2.3 输入/输出流和序列化

1) 读入有很多数据的大文件

2) 从键盘输入数字的两种方法

3) 序列化

2.4 典型的递归问题

1) 斐波那契数列

三、Java内存管理

3.1 垃圾收集

1) Java垃圾收集机制

2) Java中的内存泄露

3.2 clone

1) 特点

2) 检验类型

3) 深拷贝的定义

四、面向对象

4.1 类和对象

1) equal

2) 创建对象的几种方式

3) 嵌套类innner class

4.2 集合类

1) 分类

2) 特点

-

参考书籍：《Java面试宝典》第4版《剑指Offer》
