15道非常经典的Java面试题
本帖最后由 断天涯大虾 于 2017-3-23 10:03 编辑(一)选择题（每题5分）
(1)下面程序的运行结果是: ( )
String str1 = "hello";
String str2 = "he" + new String("llo");
System.out.println(str1 == str2);A. true B. false C. I dont know
(2) 存在使 i+1<i的数吗？（）A. yes B. no C. I dont know
(3) GC线程是否为守护线程？（）A. yes B. no C. I dont know
(4) volatile关键字是否能保证线程安全？（）A. yes B. no C. I dont know
(5) ArrayList list = new ArrayList(20);中的list扩充几次？（）A. 0 B. 1 C. 2 D. 3 E. I dont know
(6) 新建一个流对象，下面哪个选项的代码是错误的？（）A. new BufferedWriter(new FileWriter("a.txt"));B. new BufferedReader(new FileInputStream("a.dat");C. new GZIPOutputStream(new FileOutputStream("a.zip"));D. new ObjectInputStream(new FileInputStream("a.dat"));
(7) getCustomerInfo()方法如下,try中可以捕获三种类型的异常，如果在该方法运行中产生了一个IOException，将会输出什么结果（）
public void getCustomerInfo(){
try{
//do something that may cause an Exception
} catch(java.io.FileNotFoundException e){
System.out.print("FileNotFoundException");
} catch(java.io.IOException e){
System.out.print("IOException");
} catch(java.lang.Exception e){
System.out.print("Exception");
}
}A. IOExceptionB. IOExceptionExceptionC. FileNotFoundExceptionIOExceptionD. FileNotFoundExceptionIOExceptionException
(8) 下面代码的运行结果为（）
import java.io.*;
import java.util.*;
public class foo{
public static void main(String[] args){
String s;
System.out.println("s=" + s);
}
}A. 代码得到编译，并输出 "s="B. 代码得到编译，并输出 "s=null"C. 由于String s没有初始化，代码不能编译通过D. 代码得到编译,但捕获到NullPointException异常
(9) System.out.println("5"+2);的输出结果应该是（）A. 52 B. 7 C. 2 D. 5
(10) 下面的方法，当输入i为2的时候返回值是多少（）
public static int getValue(int i){
int result = 0;
switch(i){
case 1:
result = result + i;
case 2:
result = result + i*2;
case 3:
result = result + i*3;
}
return result;
}A.0 B.2 C.4 D.10
(二) 简答题
(1) 下面程序能正常运行结果为？
public static void hello(){
System.out.println("hello");
}
public static void main(String[] args){
((NULL)null).hello();
}
(2) 指出下列程序运行结果public class Example{
String str = new String("good");
char[] ch = {'a','b','c'};
public static void main(String[] args){
Example ex = new Example();
ex.change(ex.str,ex.ch);
System.out.println(ex.str + " and ");
System.out.println(ex.ch);
}
public void change(String str ,char ch[]){
str = "test ok";
ch = 'g';
}
}
(3) 下面程序的运行结果public class Test1{
public static void changeStr(String str){
str = "welcome";
}
public static void main(String[] args){
String str = "1234";
changeStr(str);
System.out.println(str);
}
}
(4) 下面程序的运行结果public class Test2{
static boolean foo(char c){
System.out.println(c);
return true;
}
public static void main(String[] args){
int i = 0;
for(foo('A'); foo('B') && (i<2); foo('C')){
i++;
foo('D');
}
}
}
(5) 下面程序的运行结果class HelloA{
public HelloA(){
System.out.println("HelloA");
}
{
System.out.println("I am A class");
}
static {
System.out.println("static A");
}
}
public calss HelloB extends HelloA{
public HelloB(){
System.out.println("HelloB");
}
{
System.out.println("I am 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 ----);
}
}
解答
/**
* socket:test.Interview.java
* 日期:2017年3月22日
*/
package test;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
* 15道非常经典的java面试题 Interview <br>
*
* @author 王俊伟 wjw.happy.love@163.com
* @blog https://www.github.com/junehappylove
* @date 2017年3月22日 上午11:02:54
* @version 1.0.0
*/
public class Interview {
/**
* 字符串不是基本的类型数据
*
*/
private static void one() {
String str1 = "hello";
String str2 = "he" + new String("llo");
System.err.println(str1 == str2);
System.out.println("1. false");
}
/**
* 考察数的范围问题
*
*/
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");
}
/**
* 这个得jdk8才能支持
*
*/
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();//不推荐这种写法，null是万物之始
Null _null = (Null) null;
_null.hello();
System.out.println("11. hello");
}
}
/**
* 形参实参
* StringExample1 <br>
*
* @author 王俊伟 wjw.happy.love@163.com
* @blog https://www.github.com/junehappylove
* @date 2017年3月22日 上午11:08:17
* @version 1.0.0
*/
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 = '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();
System.out.println("12. good and gbc");
}
}
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);
System.out.println("13. 1234");
}
}
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();
System.out.println("14. ABDCBDCB");
}
}
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");
}
}
/**
* 运行此方法测试
* @param args
*/
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);
}
}
日积月累
在泛型中，<T>和T的区别是什么呢？很多人没有回答出来。正确的答案应该是这样的：<T>是定义，T是使用，在Java中遵守先定义后使用的原则。
在泛型中，有几种分类？估计也有很多人答不出来。正确的答案是： 泛型类，泛型方法，泛型接口。
原文链接：https://my.oschina.net/junehappylove/blog/864572
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