Core Java 面试题与答案
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问题 181. Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected.
问题 182. What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.
问题 183. How can you minimize the need of garbage collection and make the memory use more effective?
Use object pooling and weak object references.
Pooling basically means utilizing the resources efficiently, by limiting access of the objects to only the period the client requires it.
Increasing utilization through pooling usually increases system performance.
Object pooling is a way to manage access to a finite set of objects among competing clients. in other words,
object pooling is nothing but sharing of objects between different clients.
Since object pooling allows sharing of objects ,the other clients/processes need to re-instantiate the object(which decreases the load time), instead they can use an existing object. After the usage , the objects are returned to the pool.
问题 184. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.
问题 185. If a class is located in a package, what do you need to change in the OS environment to be able to use it?
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:devcomxyzhrEmployee.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
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