15道非常经典的Java面试题

试题如下：

参考答案：

1import java.io.FileNotFoundException;
2import java.io.IOException;
3import java.util.ArrayList;
4import java.util.concurrent.ExecutorService;
5import java.util.concurrent.Executors;
6
7/**
8 * Created by ysc on 7/26/16.
9 */
10public class Interview {
11    private static void one(){
12        String str1 = "hello";
13        String str2 = "he"+new String("llo");
14        System.err.println(str1==str2);
15        System.out.println("1. false");
16    }
17    private static void two(){
18        int i = Integer.MAX_VALUE;
19        System.err.println((i+1)<i);
20        System.out.println("2. 存在一个i, 使得(i+1)<i");
21    }
22    private static void three(){
23        System.err.println("gc is not a Java Thread, it is a native thread");
24        Thread.getAllStackTraces().keySet().forEach(thread -> System.out.println(thread.getName()+"->"+thread.isDaemon()+" "+thread.getPriority()));
25        System.out.println("3. gc线程是daemon线程");
26    }
27    private static volatile int count = 0;
28    private static void four(){
29        ExecutorService executorService = Executors.newCachedThreadPool();
30        for(int j=0; j<10; j++){
31            executorService.submit(()->{
32                for(int i=0; i<1000000; i++){
33                    count++;
34                }
35            });
36        }
37        System.out.println("count should be: "+10000000+", actual be: "+count);
38        System.out.println("4. volatile不能保证线程安全");
39    }
40    private static void five(){
41        ArrayList<Integer> list = new ArrayList<>(20);
42        list.add(1);
43        System.out.println("debug code, not execute grow method");
44        System.out.println("5. list grow 0 times");
45    }
46    private static void six() {
47        System.out.println("BufferedReader's constructor only accepts a Reader instance");
48        System.out.println("6. new BufferedReader(new FileInputStream(\"a.dat\")); is wrong");
49    }
50    private static void seven() {
51        try{
52            if(true){
53                throw new IOException();
54            }
55        }catch (FileNotFoundException e){
56            System.out.print("FileNotFoundException!");
57        }catch (IOException e){
58            System.out.print("IOException!");
59        }catch (Exception e){
60            System.out.print("Exception!");
61        }
62        System.out.println("\n7. IOException!");
63    }
64    private static void eight() {
65        System.out.println("String s;System.out.println(s); error: variable s might not have been initialized\nRecompile with -Xlint:unchecked for details.");
66        System.out.println("8. 由于String s没有初始化, 代码不能编译通过");
67    }
68    private static void nine() {
69        System.out.println("5"+2);
70        System.out.println("9. 52");
71    }
72    private static void ten() {
73        int i = 2;
74        int result = 0;
75        switch(i){
76            case  1:
77                result = result + i;
78            case  2:
79                result = result + i * 2;
80            case  3:
81                result = result + i * 3;
82        }
83        System.out.println("result="+result);
84        System.out.println("10. 10");
85    }
86    private static class Null{
87        public static void hello(){
88            System.out.println("hello");
89        }
90        public static void main(String[] args) {
91            ((Null)null).hello();
92            Null _null = (Null)null;
93            _null.hello();
94        }
95    }
96    private static class StringExample1{
97        String str = new String("good");
98        char[] ch = {'a', 'b', 'c'};
99        public void change(String str, char[] ch){
100            str = "test ok";
101            ch[0] = 'g';
102        }
103
104        public static void main(String[] args) {
105            StringExample1 ex = new StringExample1();
106            ex.change(ex.str, ex.ch);
107            System.out.print(ex.str+" and ");
108            System.out.print(ex.ch);
109            System.out.println();
110        }
111    }
112    private static class StringExample2{
113        public static void change(String str){
114            str = "welcome";
115        }
116
117        public static void main(String[] args) {
118            String str = "1234";
119            change(str);
120            System.out.println(str);
121        }
122    }
123    private static class ForLoop{
124        static boolean foo(char c){
125            System.out.print(c);
126            return true;
127        }
128
129        public static void main(String[] args) {
130            int i=0;
131            for(foo('A');foo('B')&&(i<2);foo('C')){
132                i++;
133                foo('D');
134            }
135            System.out.println();
136        }
137    }
138    private static class HelloA{
139        public HelloA(){
140            System.out.println("HelloA");
141        }
142
143        { System.out.println("I'm A class"); }
144
145        static {
146            System.out.println("static A");
147        }
148    }
149    private static class HelloB extends HelloA{
150        public HelloB(){
151            System.out.println("HelloB");
152        }
153
154        { System.out.println("I'm B class"); }
155
156        static {
157            System.out.println("static B");
158        }
159
160        public static void main(String[] args) {
161            System.out.println("main start");
162            new HelloB();
163            new HelloB();
164            System.out.println("main end");
165        }
166    }
167    public static void main(String[] args) {
168        one();
169        two();
170        three();
171        four();
172        five();
173        six();
174        seven();
175        eight();
176        nine();
177        ten();
178        Null.main(null);
179        StringExample1.main(null);
180        StringExample2.main(null);
181        ForLoop.main(null);
182        HelloB.main(null);
183    }
184}