以下是我在上一家公司出的java笔试题，有些难度，感兴趣的同学可以做做看。
---Question---
1.What is the output of the following program?
public class Foo {
public static void main(String[] args){
Map<byte[], String> m = new HashMap<byte[], String>();
byte[] key = "abcd".getBytes();
m.put(key, "abcd");
System.out.println(m.containsKey(key));
System.out.println(m.containsKey("abcd"));
System.out.println(m.containsKey("abcd".getBytes()));
}
}
a) true,true,false b)true,false,false c)true,true,true d) false,false,false e)Program throws an exception
2. What is the proper string filled in the following program?
Public class Foo {
public static void main(String[] args) {
String s=”1\\2\\3\\4”;
//split the string with “\”
String []result = s.split(“____”);
for(String r:result){
System.out.println(r);
}
}
}
a) \ b) \\ c) \\\ d)\\\\ e)\\\\\
3. What is the output of the following program?
public class Foo {
public static void main(String[] args) {
char[] c = new char[] { '1' };
String s = new String(c);
System.out.println("abcd" + c);
System.out.println("abcd" + s);
}
}
a) Compile error b)abcd1,abcd1 c) abcd49,abcd1 d) Program throws an exception e)none of above
4. Which class is threading safe which one object can be used between multi-threads without extra synchronized?
a) Vector b) HashMap c) ArrayList d)StringBuilder e)HashSet
5. What is the output of the following program?
public class Foo {
public static void main(String[] args) throws IOException {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte[] b = new byte[]{(byte)0x0,(byte)0x1,(byte)0x2};
baos.write(b);
baos.write(0x0102);
byte[] result = baos.toByteArray();
ByteArrayInputStream bais = new ByteArrayInputStream(result);
System.out.println(bais.available());
}
}
a) 0 b) 1 c)4 d) 5 e) Program throws an exception
6. What is return value of function “calc”?
public class Foo {
public static int calc() throws IOException{
int ret = 0;
try{
++ret;
throw new IOException("try");
}
catch(IOException ioe){
--ret;
return ret;
}
finally{
++ret;
return ret;
}
}
}
a) 0 b) 1 c)2 d)3 e) throws an exception
7. What is the output of the following program?
public class Foo {
public static class Value {
private int value;
public int get(){
return value;
}
public void set(int v){
value = v;
}
}
public static class Values implements Iterable<Value>{
public Values(int capacity){
this.capacity = capacity;
}
int count =1 ;
int capacity;
Value v = new Value();
public Iterator<Value> iterator() {
return new Iterator<Value>(){
public boolean hasNext() {
return count<=capacity;
}
public Value next() {
v.set(count++);
return v;
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
}
public static void main(String[] args) {
Values vs = new Values(10);
Value result = null;
for(Value v:vs){
if(result == null){
result = v;
}
else{
result.set(result.get()+v.get());
}
}
System.out.println(result.get());
}
}
a) 20 b)40 c)45 d)55 e)throws NullpointerException
8. If add keyword “final” before a class member function, it means:
a) The method can’t access the non-final member variable.
b) The method can’t modify the member variable.
c) The method can’t be override by subclass.
d) The method is a thread-safe function.
e) The method can’t be accessed by other non-final function.
9. About java memory and garbage collector, which statement is correct?
a) Moving variable from locale to class will make GC more effectively.
b) When Full GC is executing, all the user threads will be paused.
c) If object A contains a reference of object B and object B contains a reference of object A, the two objects can’t be reclaimed by GC.
d) When a thread exits, all objects which created by that thread will be reclaimed
e) It is recommended that calling “System.gc()” to control the memory usage.
10. About java classpath and classloader, which statement is NOT correct?
a) User can specify the classpath by using the option “-cp” in Java command line.
b) If user doesn’t specify classpath, the JVM search the class from the current folder by default.
c) A JVM can load two different versions of a library.
d) To define customized class loader, it is possible to load class from internet at runtime.
11. Which data structure has best performance when remove an element from it?
a) Vector b)ArrayList c)LinkedList d)HashMap e)HashSet
12. Which is the correct way to convert bytes from charset “gb2312” to “utf-8”?
byte[] src , dst;
a) dst = new String(src,”utf-8”).getBytes(“gb2312”);
b) dst = new String(src,”gb2312”).getBytes(“utf-8”);
c) dst = new String(src,”utf-16”).getBytes();
d) dst = new String(src).getBytes();
e) None of above.