30 Java Interview Questions
*Q1. How could Java classes direct program messages to the system console, but error messages, say to a file? (怎樣改變Java程序的標準輸出輸出到一個文件，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和抽象類的區別。老生常談的問題了……)
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?(什么時候你會用synchronized 方法？什么時候會用synchronized 塊)
A. Synchronized blocks place locks for shorter periods than synchronized methods.
*Q4. Explain the usage of the keyword transient?(談一下關鍵字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?(你能強制執行垃圾回收嗎？)
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?(什么時候要做強制類型轉換？)
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() 的區別)
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? (你能寫一個程序即作為Applet又作為Application嗎？ ——經常干的
)
A. Yes. Add a main() method to the applet.
*Q9. What's the difference between constructors and other methods? (構造函數和其他的方法有什么區別)
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 (在另外一個構造函數里怎么調其他的構造函數)
A. Yes. Use this() syntax.
*Q11. Explain the usage of Java packages.(說說Package的用處)
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 有什么區別
)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()? (比較兩個String的時候用==還是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?(如果同時捕捉FileNotFoundException 和IOExceptipon catch寫的順序需要注意嗎)
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&&下面這句話會怎么樣
)
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的主要區別)
A. Java Vector class is internally synchronized and ArrayList is not.
*Q19. When should the method invokeLater()be used? (invokeLater()什么情況下會被用到？
重來沒有用到過……)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? (子類怎么調用父類的方法)
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? (隊列和堆棧有什么區別學過數據結構的都應該知道……)
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? (又是抽象類和Interface的問題)
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? (這種問題也拿來面試
)
A. Garbage collection.
**Q24. What comes to mind when someone mentions a shallow copy in Java? (當提到淺拷貝的時候，你會想到什么)
A. Object cloning.
**Q25. If you're overriding the method equals() of an object, which other method you might also consider? (在你重寫equals方法的時候，你還會想到重寫什么方法)
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? (你打算對一些Object進行索引搜索，你會用ArrayList還是LinkedList
)
A. ArrayList
**Q27. How would you make a copy of an entire Java object with its state? (怎樣才能克隆一個Java對象)
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? (怎樣才能需要最少的GC以及最大效率的使用內存)
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， 你應該怎么做？)
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? (怎樣定義default的訪問級別)A. You do not need to specify any access level, and Java will use a default package access level.
總的來說老外的Java面試體太簡單了……讓人昏昏欲睡啊……
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