30道Java 1.4模拟经典题(1)
1. What will happen when you attempt to compile and run the following code?
(Assume that the code is compiled and run with assertions enabled.)
public class AssertTest{
public void methodA(int i){
assert i >= 0 : methodB();
System.out.println(i);
}
public void methodB(){? //无返回值
System.out.println(”The value must not be negative”);
}
public static void main(String args[]){
AssertTest test = new AssertTest();
test.methodA(-10);
}
}
A.it will print -10
B.it will result in AssertionError showing the message-“the value must not be negative”.
C.the code will not compile.
D.None of these.
C is correct. An assert statement can take any one of these two forms -
assert Expression1;
assert Expression1 : Expression2;
Note that, in the second form; the second part of the statement must be an expression- Expression2. In this code, the methodB() returns void, which is not an expression and hence it results in a compile time error. The code will compile if methodB() returns any value such as int, String etc.
Also, in both forms of the assert statement, Expression1 must have type boolean or a compile-time error occurs.
2. What will happen when you attempt to compile and run the following code?
public class Static{
static{
int x = 5;? //在static内有效
}
static int x,y;?? //初始化为0
public static void main(String args[]){
?????? x-;?? //-1
myMethod();
?????? System.out.println(x + y + ++x);
}
public static void myMethod(){
y = x++ + ++x;? //y=-1+1? x=1
}
}
A.compiletime error
B.prints: 1
C.prints: 2
D.prints: 3
E.prints: 7
F.prints: 8
D is the correct choice. The above code will not give any compilation error. Note that “Static” is a valid class name. Thus choice A is incorrect.
In the above code, on execution, first the static variables (x and y) will be initialized to 0. Then static block will be called and finally main() method will be called. The execution of static block will have no effect on the output as it declares a new variable (int x).
The first statement inside main (x-) will result in x to be -1. After that myMethod() will be executed. The statement “y = x++ + ++x;” will be evaluated to y = -1 + 1 and x will become 1. In case the statement be “y =++x + ++x”, it would be evaluated to y = 0 + 1 and x would become 1. Finally when System.out is executed “x + y + ++x” will be evaluated to “1 + 0 + 2″ which result in 3 as the output. Thus choice D is correct.
3. Given the following code, what will be the output?
class Value{
public int i = 15;
}
public class Test{
public static void main(String argv[]){
?????? Test t = new Test();
t.first();
?? }
public void first(){
?????? int i = 5;
?????? Value v = new Value();
v.i = 25;
second(v, i);
?? System.out.println(v.i);
}
public void second(Value v, int i){
i = 0;
?????? v.i = 20;
Value val = new Value();
?????? v = val;
?? System.out.println(v.i + ” ” + i);
}
}
A.15 0 20
B.15 0 15
C.20 0 20
D.0 15 20
A is correct. When we pass references in Java what actually gets passed is the value of that reference (i.e. memory address of the object being referenced and not the actual object referenced by that reference) and it gets passed as value (i.e a copy of the reference is made). Now when we make changes to the object referenced by that reference it reflects on that object even outside of the method being called but any changes made to the reference itself is not reflected on that reference outside of the method which is called. In the example above when the reference v is passed from method first() to second() the value of v is passed. When we assign the value val to v it is valid only inside the method second() and thus inside the method second() what gets printed is 15 (initial value of i in the object referenced by val), then a blank space and then 0 (value of local variable i). After this when we return to the method first() v actually refers to the same object to which it was referring before the method second() was called, but one thing should be noted here that the value of i in that object (referred by v inside the method first()) was changed to 20 in the method second() and this change does reflect even outside the method second(), hence 20 gets printed in the method first(). Thus overall output of the code in consideration is
15 0
20
4. What will happen when you attempt to compile and run the following code?
class MyParent {
int x, y;
MyParent(int x, int y){
this.x = x;
this.y = y;
}
public int addMe(int x, int y){
return this.x + x + y + this.y;
}
public int addMe(MyParent myPar){
return addMe(myPar.x, myPar.y);
}
}
class MyChild extends MyParent{
int z;
MyChild (int x, int y, int z){
super(x,y);
this.z = z;
}
public int addMe(int x, int y, int z){
return this.x + x + this.y + y + this.z + z;
}
public int addMe(MyChild myChi){
return addMe(myChi.x, myChi.y, myChi.z);
}
public int addMe(int x, int y){
return this.x + x + this.y + y;
}
}
public class MySomeOne{
public static void main(String args[]){
MyChild myChi = new MyChild(10, 20, 30);
MyParent myPar = new MyParent(10, 20);
int x = myChi.addMe(10, 20, 30);
int y = myChi.addMe(myChi);
int z = myPar.addMe(myPar);
System.out.println(x + y + z);
}
}
A.300
B.240
C.120
D.180
E.compile error
F.none of the above
A is the correct choice. In the above code, MyChild class overrides the addMe(int x, int y) method of the MyParent class. And in both the MyChild and MyParent class, addMe() method is overloaded. There is no compilation error anywhere in the above code.
On execution, first, the object of MyChild class will be constructed. Please note that there is a super() call from the constructor of MyChild class, which will call the constructor of MyParent class. This will cause the value of z variable of MyChild class to be 30 and x, y variables of MyParent class will become 10 and 20 respectively. The next statement will again call the constructor of MyParent class with same x and y values. This is followed by execution of addMe() method of MyChild class with x as 10, y as 20 and z as 30. Also x and y are inherited by MyChild class from the MyParent class. Thus in the addMe() method of the MyChild class, the value of this.x will be 10, this.y will be 20 and this.z will be 30. The return value of this method will be “10 + 10 + 20 + 20 + 30 + 30″, which is equal to 120. Thus x will become 120.
This is followed by the invocation of the other addMe() method which takes object reference of the MyChild class. From this method, the method which was called earlier is invoked. This call is exactly the same as the earlier one. Thus the value of y will also be 120 like x.
Now the addMe() method of MyParent class is invoked. This method invokes another addMe() method of the same class. Its equivalent to the invocation of addMe(int x, int y) method with x as 10 and y as 20. Also the value of instance variables x and y of My Parent class is 10 and 20 respectively. The value of z will be evaluated to “10 + 10 + 20 + 20″, which is equal to 60. Thus the value of x, y and z after all the invocations will be 120, 120 and 60 respectively. As a result of this finally, “120 + 120 + 60″ which is equal to 300 will be printed. Thus A is the correct choice.
5. The class AssertionError has “is -a” relationship with these classes (choose two)
A.RuntimeException
B.Error
C.VirtualMachineError
D.IllegalAccessException
E.Throwable
B and E are correct. The class AssertionError is an Error, which denotes an “incorrect condition” as opposed to an “unusual condition” (Exception).? Since, the class Error descends from Throwable, AssertionError also has “is-a” relationship with Throwable. Here is the hierarchy ?Cjava.lang.Object
?|
?+-java.lang.Throwable
?????? |
?????? +-java.lang.Error
???????????? |
???????????? +-java.lang.AssertionError
Want to know more?
You can find more information about this as an answer to the question - “Why is AssertionError a subclass of Error rather than RuntimeException?” at - http://java.sun.com/j2se/1.4/docs/guide/lang/assert.html #design-faq-error
6. What will be the result of executing the following code?
1. boolean a = true;
2. boolean b = false;
3. boolean c = true;
4. if (a == true)
5.?? if (b == true)
6.?????? if (c == true) System.out.println(”Some things are true in this world”);
7.?????? else System.out.println(”Nothing is true in this world!”);
8.?? else if (a && (b = c))?? //这里是赋值，不是比较
System.out.println(”It's too confusing to tell what is true and what is false”);
9.?? else? System.out.println(”Hey this won't compile”);
A.The code won’t compile.
B.“some things are true in this world” will be printed
C.“hey this won’t compile” will be printed
D.None of these
D is correct. This is a very good question to test the concepts of execution flow in case of if conditions. The rule for attaching else statements with if conditions is the same as attaching close brackets with open brackets. A close bracket attaches with the closest open bracket, which is not already closed. Similarly an else statement attaches with the closest if statement, which doesn't have an else statement already, attached to it. So the else statement at line 7 attaches to the if statement at line 6. The else statement at line 8 attaches to the if statement at line 5. The else statement at line 9 attaches to the if statement at line 8.
Now let's look at the execution. At line 4 since a is equal to true the execution falls to line 5. At line 5 since b is not true the execution goes to the corresponding else statement at line 8. Now it evaluates the condition inside the if statement. Please note here that an assignment statement also has a value equal to the value being assigned, hence (b = c) evaluates to true and subsequently a && (b = c) evaluates to true and “It's too confusing to tell what is true and what is false” will be printed. Hence the correct answer is choice D.
7. What will happen when you attempt to compile and run the following code?
interface MyInterface{}
public class MyInstanceTest implements MyInterface{
static String s;
public static void main(String args[]){
MyInstanceTest t = new MyInstanceTest();
if(t instanceof MyInterface){
System.out.println(”I am true interface”);
}else {
System.out.println(”I am false interface”);
}
if(s instanceof String){
System.out.println(”I am true String”);
}else {
System.out.println(”I am false String”);
}
}
}
A.compile time error
B.runtime error
C.prints: “I am true interface” followed by “I am true String”
D.prints: “I am false interface” followed by “I am false String”
E.prints: “I am true interface” followed by “I am false String”
F.prints: “I am false interface” followed by “I am true String”
E is the correct choice. The “instanceof” operator tests the class of an object at
runtime. It returns true if the class of the left-hand argument is the same as, or
is some subclass of, the class specified by the right-hand operand.
The right-hand operand may equally well be an interface. In such a case, the test
?determines if the object at left-hand argument implements the specified interface.
In the above case there will not be any compiletime or runtime error. The result of
?”t instance of MyInterface” will be true as “t” is the object of MyInstanceTest
?class which implements the MyInstance interface. But the result of “s instanceof
?String” will be false as “s” refers to null. Thus the output of the above program
?will be : “I am true interface” followed by ” I am false String”. Thus choice E is
?correct and others are incorrect.
8. What results from attempting to compile and run the following code?
public class Ternary{
public static void main(String args[]){
int a = 5;
System.out.println(”Value is - ” + ((a < 5) ? 9.9 : 9));
}
}
A.print:Value is -9
B.print:Value is -5
C.Compilation error
D.None of these
D is correct. The code compiles successfully. In this code the optional value for
the ternary operator, 9.0(a double) and 9(an int) are of different types. The
?result of a ternary operator must be determined at the compile time, and here the
? type chosen using the rules of promotion for binary operands, is double.
?? Since the result is a double, the output value is printed in a floating point
?? format. The choice of which value to be printed is made on the basis of the
?? result of the comparison “a < 5″ which results in false, hence the variable “a”
?? takes the second of the two possible values, which is 9, but because the result
?? type is promoted to double, the output value is actually written as 9.0, rather
?? than the more obvious 9, hence D is correct.
9. In the following pieces of code,
A and D will compile without any error. True/False?
A: StringBuffer sb1 = “abcd”;
B: Boolean b = new Boolean(”abcd”);
C: byte b = 255;
D: int x = 0×1234;
E: float fl = 1.2;
True
False
The code segments B and D will compile without any error.
?A is not a valid way to construct a StringBuffer, you need to creat
? a StringBuffer object using “new”.
? B is a valid construction of a Boolean
? (any string other than “true” or “false” to the Boolean constructor will
? result in a Boolean with a value of “false”).
?? C will fail to compile because the valid range for a byte is -128 to +127 (
?? ie, 8 bits,signed). D is correct, 0×1234 is the hexadecimal representation
?? in java. E fails to compile because the compiler interprets 1.2 as
?? a double being assigned to a float (down-casting), which is not valid.
?? You either need an explicit cast (as in “(float)1.2″) or “1.2f”,
??? to indicate a float.
10. Considering the following code, Which variables may be referenced correctly
at line 12?
1.public class Outer
2.{
3.public int a = 1;
4.private int b = 2;
5.public void method(final int c)
6.{
7.int d = 3;
8.class Inner
9.{
10.private void iMethod(int e)
11. {
12.
13.}
14.}
15.}
16.}
a b c d e
A, B, C and E are correct. Since Inner is not a static inner class,
it has a reference to an enclosing object, and all the variables of that object are
?accessible. Therefore A and B are correct, even if b is private.
? Variables in the enclosing method are only accessible when they are marked as
? final hence c is accessible but not d. E is obviously
? correct as it is a parameter to the method containing line 12 itself.
11. What will be the result of executing the following code?
public static void main(String args[]){
?? char digit = ‘a';
?? for (int i = 0; i < 10; i++){
???? switch (digit){
?????? case ‘x' :{
?????? int j = 0;
?? System.out.println(j);
?????? }
?????? default :{
?????? int j = 100;
?? System.out.println(j);
?????? }
???? }
? }
? int i = j;
? System.out.println(i);
}
A.100 will be printed 11 times.
B.100 will be printed 10 times and then there will be a runtime exception
C.The code will not compile because the variable i cannot be declared twice within the mani() method.
D.The code will not compile because the variable j cannot be declared twice within the switch statement.
E.None of these.
E is correct. The code will not compile. There is no problem with the declaration
of another variable i as both the variables are in disjoint blocks
(first one is inside the for loop and its scope ends with the for loop,
whereas the second is outside the for loop) and,
therefore, different scopes and hence valid. The problem is with the variable j.
The two declarations of the variable j are perfectly valid as they are in disjoint
?blocks and, therefore, different scopes. The error is that both the declarations
?of j are not available outside the case or default statement,
?whereas we are trying to assign it to the variable i.
?Therefore the compiler objects and reports variable j not found.
12. Which of the following collection classes from java.util package are Thread safe?
A.Vector
B.ArrayList? //与Vector类似，只是不同步
C.HashMap
D.Hashtable
A and D are correct. Vector and Hashtable are two collection classes that are inherently thread safe or synchronized; whereas, the classes ArrayList and HashMap are unsynchronized and must be “wrapped” via Collections.SynchronizedList or Collections.synchronizedMap if synchronization is desired.
13. What will happen when you attempt to compile and run the following code?
class MyThread extends Thread{
public void run(){
System.out.println(”MyThread: run()”);
}
public void start(){
System.out.println(”MyThread: start()”);
}
}
class MyRunnable implements Runnable{
public void run(){
System.out.println(”MyRunnable: run()”);
}
public void start(){
System.out.println(”MyRunnable: start()”);
}
}
public class MyTest {
public static void main(String args[]){
MyThread myThread? =? new MyThread();
MyRunnable myRunnable = new MyRunnable();
Thread thread? =? new Thread(myRunnable);
myThread.start();
thread.start();
}
}
A.prints: MyThread: start() followed by MyRunnable: run()
B.prints: MyThread: run() followed by MyRunnable: start()
C.prints: MyThread: start() followed by MyRunnable: start()
D.prints: MyThread: run() followed by MyRunnable: run()
E.compile time error
F.None of the above
A is the correct choice. In the above code there is not any compilation error. Thus choice E is incorrect. Inside main() method, objects of MyThread and MyRunnable class are created followed by creation of? Thread with object of MyRunnable class.
Note that MyThread class extends Thread class and overrides the start() method of the Thread class. Thus on execution of “myThread.start()” statement, the start() method of the MyThread class will be executed and as a result “MyThread:start()” will be printed. Had the start() method not there in MyThread class, the start() method of the Thread class would be called which in turn would call the run() method of the MyThread class.
On execution of “thread.start();”, the start() method of the Thread class would be called which in turn will call the run() method of the class which is passed to Thread constructor (i.e. MyRunnable class). Thus “MyRunnable:run()” will be printed out. Thus choice A is correct.
14. What will be the result of executing the following code?
// Filename; SuperclassX.java
package packageX;
public class SuperclassX{
protected void superclassMethodX(){}
int superclassVarX;
}
// Filename SubclassY.java
1.package packageX.packageY;
2.
3.public class SubclassY extends SuperclassX
4.{
5.SuperclassX objX = new SubclassY();
6.SubclassY objY = new SubclassY();
7.void subclassMethodY()
8.{
9.objY.superclassMethodX();
10.int i;
11.i = objY.superclassVarX;
12.}
13.}
A.Compile error at line 5.
B.Compile error at line 9.
C.Runtime exception at line 11.
D.None of these
D is correct. When no access modifier is specified for a member, it is only accessible by another class in the package where its class is defined. Even if its class is visible in another package, the member is not accessible there. In the question above the variable superclassVarX has no access modifier specified and hence it cannot be accessed in the packageY even though the class SuperclassX is visible and the protected method superclassMethodX() can be accessed. Thus the compiler will raise an error at line 11.
15. Consider the class hierarchy shown below:
???? FourWheeler
(implements DrivingUtilities)
/ / \ \
?? /? /?? \? \
?/?? /???? \?? \
/??? /?????? \??? \
?? /???? /???????? \???? \
?Car? Truck??????? Bus? Crane
Consider the following code below:
1.DrivingUtilities du;
2.FourWheeler fw;
3.Truck myTruck = new Truck();
4.du = (DrivingUtilities)myTruck;
5.fw = new Crane();
6.fw = du;
Which of the statements below are true?
A.Line 4 will not compile because an interface cannot refer to an object.
B.The code will compile and run.
C.The code will not compile without an explicit cast at line 6, because going down the hierarchy without casting is not allowed.
D.The code at line 4 will compile even without the explicit cast.
E.The code will compile if we put an explicit cast at line 6 but will throw an exception at runtime.
C and D are correct. A and B are obviously wrong because there is nothing wrong in an interface referring to an object. C is correct because an explicit cast is needed to go down the hierarchy. D is correct because no explicit cast is needed at line 4, because we are going up the hierarchy. E is incorrect because if we put an explicit cast at line 6, the code will compile and run perfectly fine, no exception will be thrown because the runtime class of du (that is Truck) can be converted to type FourWheeler without any problem.
