101.
Question :What will be the output of the following statement?
System.out.println ("1" + 3);
102. Question :What will be the default values of all the elements of an array defined as an instance variable?
103. Question: How could Java classes direct program messages to the system console, but error messages, say to a file?
104. Question: What's the difference between an interface and an abstract class?
105. Question: Why would you use a synchronized block vs. synchronized method?
106. Question: Explain the usage of the keyword transient?
107. Question: How can you force garbage collection?
108. Question: How do you know if an explicit object casting is needed?
109. Question: What's the difference between the methods sleep() and wait()
110. Question: Can you write a Java class that could be used both as an applet as well as an application?
111. Question: What's the difference between constructors and other methods?
112. Question: Can you call one constructor from another if a class has multiple constructors
113. Question: Explain the usage of Java packages.
114. Question: If a class is located in a package, what do you need to change in the OS environment to be able to use it?
115. Question: What's the difference between J2SDK 1.5 and J2SDK 5.0?
116. Question: What would you use to compare two String variables - the operator == or the method equals()?
117. Question: Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
118. Question: Can an inner class declared inside of a method access local variables of this method?
119. Question: What can go wrong if you replace && with & in the following code: String a=null; if (a!=null && a.length()>10) {...}
120. Question: What's the main difference between a Vector and an ArrayList
121. Question: When should the method invokeLater()be used?
122. Question: How can a subclass call a method or a constructor defined in a superclass?
123. Question: What's the difference between a queue and a stack?
Answer :
It will print 13102. Question :What will be the default values of all the elements of an array defined as an instance variable?
Answer :
If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null103. Question: How could Java classes direct program messages to the system console, but error messages, say to a file?
Answer :
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);104. Question: What's the difference between an interface and an abstract class?
Answer :
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 class105. Question: Why would you use a synchronized block vs. synchronized method?
Answer :
Synchronized blocks place locks for shorter periods than synchronized methods106. Question: Explain the usage of the keyword transient?
Answer :
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)107. Question: How can you force garbage collection?
Answer :
You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately108. Question: How do you know if an explicit object casting is needed?
Answer :
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 automatically109. Question: What's the difference between the methods sleep() and wait()
Answer :
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. Wait can be called only from synchronize Wait be release lock Object class method while sleep in thread method110. Question: Can you write a Java class that could be used both as an applet as well as an application?
Answer :
Yes. Add a main() method to the applet111. Question: What's the difference between constructors and other methods?
Answer :
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 times112. Question: Can you call one constructor from another if a class has multiple constructors
Answer :
Yes. Use this() syntax113. Question: Explain the usage of Java packages.
Answer :
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 classes114. Question: If a class is located in a package, what do you need to change in the OS environment to be able to use it?
Answer :
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.Employee115. Question: What's the difference between J2SDK 1.5 and J2SDK 5.0?
Answer :
There's no difference, Sun Microsystems just re-branded this version116. Question: What would you use to compare two String variables - the operator == or the method equals()?
Answer :
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 object117. Question: Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
Answer :
Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first118. Question: Can an inner class declared inside of a method access local variables of this method?
Answer :
It's possible if these variables are final119. Question: What can go wrong if you replace && with & in the following code: String a=null; if (a!=null && a.length()>10) {...}
Answer :
A single ampersand here would lead to a NullPointerException120. Question: What's the main difference between a Vector and an ArrayList
Answer :
Java Vector class is internally synchronized and ArrayList is not121. Question: When should the method invokeLater()be used?
Answer :
This method is used to ensure that Swing components are updated through the event-dispatching thread122. Question: How can a subclass call a method or a constructor defined in a superclass?
Answer :
Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor123. Question: What's the difference between a queue and a stack?
Answer :
Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule
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