Java面试题（1）－ 高级特性

1. The diffrence between java.lang.StringBuffer and java.lang.StringBuilder?

java.lang.StringBuffer: thread-safe, synchronized and not so faster.

java.lang.StringBuilder: faster, performs no synchronization.

2. How to handle uncaught Exception for a thread?

@FunctionalInterface
public static interface Thread.UncaughtExceptionHandler

3. The difference between [checked exceptions] and [unchecked exceptions]?

The class Exception and any subclasses that are not also subclasses of RuntimeException are checked exceptions. Checked exceptions need to be declared in a method or constructor's throws clause if they can be thrown by the execution of the method or constructor and propagate outside the method or constructor boundary.

Checked Exceptions should be used for expected, but unpreventable errors that are reasonable to recover from.

4. How to use non static innner class and static inner class?

non static: Out.In in = new Out().new In();

static: Out.StaticIn in = new Out.StaticIn();

5. Anonymous Inner Class?

Anonymous inner class must extends a super class or implements an interface.

ClassA a = new ClassA();

a.test(new Product(){

　　public double process() {

　　}

});

6. How to create dynamic proxy class and dynamic proxy instance?

java.lang.reflect.Proxy
interface InvocationHandler

static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces)
static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)

InvocationHandler handler = new MyInvocationHandler(...);

Class proxyClass = Proxy.getProxyClass(Foo.class.getClassLoader(), new Class[] { Foo.class });
Constructor ctor = proxyClass.getConstructor(new Class[] { InvocationHandler.class });
Foo f = (Foo)ctor.newInstance(new Object[] { handler });

Foo f = (Foo)Proxy.newProxyInstance(Foo.class.getClassLoader(), new Class[] { Foo.class }, handler);

7. What's the difference between 3 types of class loaders?

public abstract class ClassLoader extends Object

loadClass(String name, boolean resolve)

findClass(String name)

Bootstrap ClassLoader: load core classes of Java. (java.exe -Xbootclasspath can load additional class). Not a subclass of java.lang.ClassLoader.

jre/lib/*.jar

jre/classes

Extension ClassLoader: jre/lib/ext/* or defined by java.ext.dirs System Property.

System ClassLoader: java -classpath or defined by java.class.path

java.lang.Object
　　java.lang.ClassLoader
　　　　java.security.SecureClassLoader
　　　　　　java.net.URLClassLoader

Parents of Extension ClassLoader and System ClassLoader.

8. How does interface Serializable works?

All subtypes of a serializable class are themselves serializable.

Classes that require special handling during the serialization and deserialization:

private void writeObject(java.io.ObjectOutputStream out) throws IOException
private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException;
private void readObjectNoData() throws ObjectStreamException;

Designate an alternative object to be used when writing an object to the stream:

ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException;

Designate a replacement when read from the stream:

ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;

ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;

9. How does Buffer works?

java.nio.Buffer
capacity is the number of elements it contains.
limit is the index of the first element that should not be read or written.
position is the index of the next element to be read or written.
mark is the index to which its position will be reset when the reset method is invoked.

0 <= mark <= position <= limit <= capacity

clear() sets the limit to the capacity and the position to zero.

flip() sets the limit to the current position and then sets the position to zero.

rewind() leaves the limit unchanged and sets the position to zero.

Buffers are not safe for use by multiple concurrent threads.

b.flip().position(23).limit(42);

10. The difference between interfaces of runnable and callable<v>?

callable can have return value.

11. The difference between java.util.Collections and interface Map<K,V>?

Collections contains only items while Map contains key-value pairs.

12. The difference between interface Set<E> (java.util.HashSet<E>) and interface List<E> (java.util.ArrayList<E>)?

Unordered and No-duplicated for Set<E>, Ordered and duplicated for List<E>.

13. The difference between HashSet, TreeSet and EnumSet?

14. The difference between HashTable, HashMap, EnumMap and TreeMap?

Non thread safe, key and value can be null for HashMap.

Thread safe, key and value can't be null for HashTable.