30 Java Interview Questions

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30 Java Interview Questions

*Q1. How could Java classes direct program messages to the system console, but error messages, say to a file? (怎样改变Java程序的标准输出输出到一个文件，而不是console)

A. The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:
Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);


*Q2. What's the difference between an interface and an abstract class?(Interface和抽象类的区别。老生常谈的问题了……)

A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.


*Q3. Why would you use a synchronized block vs. synchronized method?(什么时候你会用synchronized 方法？什么时候会用synchronized 块)

A. Synchronized blocks place locks for shorter periods than synchronized methods.


*Q4. Explain the usage of the keyword transient?(谈一下关键字transient的作用)

A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).


*Q5. How can you force garbage collection?(你能强制执行垃圾回收吗？)

A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.


*Q6. How do you know if an explicit object casting is needed?(什么时候要做强制类型转换？)

A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;

When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.


*Q7. What's the difference between the methods sleep() and wait() (sleep() 和 wait() 的区别)

A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.


*Q8. Can you write a Java class that could be used both as an applet as well as an application? (你能写一个程序即作为Applet又作为Application吗？ ——经常干的)

A. Yes. Add a main() method to the applet.


*Q9. What's the difference between constructors and other methods? (构造函数和其他的方法有什么区别)

A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.


*Q10. Can you call one constructor from another if a class has multiple constructors (在另外一个构造函数里怎么调其他的构造函数)

A. Yes. Use this() syntax.


*Q11. Explain the usage of Java packages.(说说Package的用处)

A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.


*Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?

A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee


*Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0? (J2SDK 1.5 和 J2SDK 5.0 有什么区别)

A.There's no difference, Sun Microsystems just re-branded this version.


*Q14. What would you use to compare two String variables - the operator == or the method equals()? (比较两个String的时候用==还是equals()方法)

A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.


*Q15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?(如果同时捕捉FileNotFoundException 和IOExceptipon catch写的顺序需要注意吗)

A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.


*Q16. Can an inner class declared inside of a method access local variables of this method?

A. It's possible if these variables are final.


*Q17. What can go wrong if you replace && with & in the following code: (如果不小心用&代替了&&下面这句话会怎么样)
String a=null; if (a!=null && a.length()>10) {...}
A. A single ampersand here would lead to a NullPointerException.


*Q18. What's the main difference between a Vector and an ArrayList (Vector和ArrayList的主要区别)

A. Java Vector class is internally synchronized and ArrayList is not.



*Q19. When should the method invokeLater()be used? (invokeLater()什么情况下会被用到？重来没有用到过……)

A. This method is used to ensure that Swing components are updated through the event-dispatching thread.


*Q20. How can a subclass call a method or a constructor defined in a superclass? (子类怎么调用父类的方法)

A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.


For senior-level developers:

**Q21. What's the difference between a queue and a stack? (队列和堆栈有什么区别学过数据结构的都应该知道……)

A. Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule


**Q22. You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces? (又是抽象类和Interface的问题)

A. Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.


**Q23. What comes to mind when you hear about a young generation in Java? (这种问题也拿来面试)

A. Garbage collection.


**Q24. What comes to mind when someone mentions a shallow copy in Java? (当提到浅拷贝的时候，你会想到什么)

A. Object cloning.


**Q25. If you're overriding the method equals() of an object, which other method you might also consider? (在你重写equals方法的时候，你还会想到重写什么方法)

A. hashCode()


**Q26. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use:
ArrayList or LinkedList? (你打算对一些Object进行索引搜索，你会用ArrayList还是LinkedList)

A. ArrayList


**Q27. How would you make a copy of an entire Java object with its state? (怎样才能克隆一个Java对象)

A. Have this class implement Cloneable interface and call its method clone().


**Q28. How can you minimize the need of garbage collection and make the memory use more effective? (怎样才能需要最少的GC以及最大效率的使用内存)

A. Use object pooling and weak object references.


**Q29. There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it? (class A 要通知 class B， 你应该怎么做？)

A. If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.


*Q30. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it? (怎样定义default的访问级别)

A. You do not need to specify any access level, and Java will use a default package access level.