15道非常经典的Java面试题 附详细答案

试题如下：

参考答案：

import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * Created by ysc on 7/26/16.
 */
public class Interview {
 private static void one(){
 String str1 = "hello";
 String str2 = "he"+new String("llo");
 System.err.println(str1==str2);
 System.out.println("1. false");
 }
 private static void two(){
 int i = Integer.MAX_VALUE;
 System.err.println((i+1)<i);
 System.out.println("2. 存在一个i, 使得(i+1)<i");
 }
 private static void three(){
 System.err.println("gc is not a Java Thread, it is a native thread");
 Thread.getAllStackTraces().keySet().forEach(thread -> System.out.println(thread.getName()+"->"+thread.isDaemon()+" "+thread.getPriority()));
 System.out.println("3. gc线程是daemon线程");
 }
 private static volatile int count = 0;
 private static void four(){
 ExecutorService executorService = Executors.newCachedThreadPool();
 for(int j=0; j<10; j++){
  executorService.submit(()->{
  for(int i=0; i<1000000; i++){
   count++;
  }
  });
 }
 System.out.println("count should be: "+10000000+", actual be: "+count);
 System.out.println("4. volatile不能保证线程安全");
 }
 private static void five(){
 ArrayList<Integer> list = new ArrayList<>(20);
 list.add(1);
 System.out.println("debug code, not execute grow method");
 System.out.println("5. list grow 0 times");
 }
 private static void six() {
 System.out.println("BufferedReader's constructor only accepts a Reader instance");
 System.out.println("6. new BufferedReader(new FileInputStream(\"a.dat\")); is wrong");
 }
 private static void seven() {
 try{
  if(true){
  throw new IOException();
  }
 }catch (FileNotFoundException e){
  System.out.print("FileNotFoundException!");
 }catch (IOException e){
  System.out.print("IOException!");
 }catch (Exception e){
  System.out.print("Exception!");
 }
 System.out.println("\n7. IOException!");
 }
 private static void eight() {
 System.out.println("String s;System.out.println(s); error: variable s might not have been initialized\nRecompile with -Xlint:unchecked for details.");
 System.out.println("8. 由于String s没有初始化, 代码不能编译通过");
 }
 private static void nine() {
 System.out.println("5"+2);
 System.out.println("9. 52");
 }
 private static void ten() {
 int i = 2;
 int result = 0;
 switch(i){
  case 1:
  result = result + i;
  case 2:
  result = result + i * 2;
  case 3:
  result = result + i * 3;
 }
 System.out.println("result="+result);
 System.out.println("10. 10");
 }
 private static class Null{
 public static void hello(){
  System.out.println("hello");
 }
 public static void main(String[] args) {
  ((Null)null).hello();
  Null _null = (Null)null;
  _null.hello();
 }
 }
 private static class StringExample1{
 String str = new String("good");
 char[] ch = {'a', 'b', 'c'};
 public void change(String str, char[] ch){
  str = "test ok";
  ch[0] = 'g';
 }

 public static void main(String[] args) {
  StringExample1 ex = new StringExample1();
  ex.change(ex.str, ex.ch);
  System.out.print(ex.str+" and ");
  System.out.print(ex.ch);
  System.out.println();
 }
 }
 private static class StringExample2{
 public static void change(String str){
  str = "welcome";
 }

 public static void main(String[] args) {
  String str = "1234";
  change(str);
  System.out.println(str);
 }
 }
 private static class ForLoop{
 static boolean foo(char c){
  System.out.print(c);
  return true;
 }

 public static void main(String[] args) {
  int i=0;
  for(foo('A');foo('B')&&(i<2);foo('C')){
  i++;
  foo('D');
  }
  System.out.println();
 }
 }
 private static class HelloA{
 public HelloA(){
  System.out.println("HelloA");
 }

 { System.out.println("I'm A class"); }

 static {
  System.out.println("static A");
 }
 }
 private static class HelloB extends HelloA{
 public HelloB(){
  System.out.println("HelloB");
 }

 { System.out.println("I'm B class"); }

 static {
  System.out.println("static B");
 }

 public static void main(String[] args) {
  System.out.println("main start");
  new HelloB();
  new HelloB();
  System.out.println("main end");
 }
 }
 public static void main(String[] args) {
 one();
 two();
 three();
 four();
 five();
 six();
 seven();
 eight();
 nine();
 ten();
 Null.main(null);
 StringExample1.main(null);
 StringExample2.main(null);
 ForLoop.main(null);
 HelloB.main(null);
 }
}

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