Java.lang Class problems
- 1. What will be the output of the program?
interface Foo141 { int k = 0; /* Line 3 */ } public class Test141 implements Foo141 { public static void main(String args[]) { int i; Test141 test141 = new Test141(); i = test141.k; /* Line 11 */ i = Test141.k; i = Foo141.k; } }
Options- A. Compilation fails.
- B. Compiles and runs ok.
- C. Compiles but throws an Exception at runtime.
- D. Compiles but throws a RuntimeException at runtime. Discuss
Correct Answer: Compiles and runs ok.
Explanation:
The variable k on line 3 is an interface constant, it is implicitly public, static, and final. Static variables can be referenced in two ways:
Via a reference to any instance of the class (line 11)
Via the class name (line 12).
- 2. What will be the output of the program?
class Q207 { public static void main(String[] args) { int i1 = 5; int i2 = 6; String s1 = "7"; System.out.println(i1 + i2 + s1); /* Line 8 */ } }
Options- A. 18
- B. 117
- C. 567
- D. Compiler error Discuss
Correct Answer: 117
Explanation:
This question is about the + (plus) operator and the overriden + (string cocatanation) operator. The rules that apply when you have a mixed expression of numbers and strings are:
If either operand is a String, the + operator concatenates the operands.
If both operands are numeric, the + operator adds the operands.
The expression on line 6 above can be read as "Add the values i1 and i2 together, then take the sum and convert it to a string and concatenate it with the String from the variable s1". In code, the compiler probably interprets the expression on line 8 above as:
System.out.println( new StringBuffer()
.append(new Integer(i1 + i2).toString())
.append(s1)
.toString() );
- 3. What will be the output of the program?
String s = "hello"; Object o = s; if( o.equals(s) ) { System.out.println("A"); } else { System.out.println("B"); } if( s.equals(o) ) { System.out.println("C"); } else { System.out.println("D"); }- A
- B
- C
- D
Options- A. 1 and 3
- B. 2 and 4
- C. 3 and 4
- D. 1 and 2 Discuss
Correct Answer: 1 and 3
- 4. What will be the output of the program?
public class ObjComp { public static void main(String [] args ) { int result = 0; ObjComp oc = new ObjComp(); Object o = oc; if (o == oc) result = 1; if (o != oc) result = result + 10; if (o.equals(oc) ) result = result + 100; if (oc.equals(o) ) result = result + 1000; System.out.println("result = " + result); } }
Options- A. 1
- B. 10
- C. 101
- D. 1101 Discuss
Correct Answer: 1101
Explanation:
Even though o and oc are reference variables of different types, they are both referring to the same object. This means that == will resolve to true and that the default equals() method will also resolve to true.
- 5. What will be the output of the program?
public class Example { public static void main(String [] args) { double values[] = {-2.3, -1.0, 0.25, 4}; int cnt = 0; for (int x=0; x < values.length; x++) { if (Math.round(values[x] + .5) == Math.ceil(values[x])) { ++cnt; } } System.out.println("same results " + cnt + " time(s)"); } }
Options- A. same results 0 time(s)
- B. same results 2 time(s)
- C. same results 4 time(s)
- D. Compilation fails. Discuss
Correct Answer: same results 2 time(s)
Explanation:
Math.round() adds .5 to the argument then performs a floor(). Since the code adds an additional .5 before round() is called, it's as if we are adding 1 then doing a floor(). The values that start out as integer values will in effect be incremented by 1 on the round() side but not on the ceil() side, and the noninteger values will end up equal.
- 6. What will be the output of the program?
class Tree { } class Pine extends Tree { } class Oak extends Tree { } public class Forest1 { public static void main (String [] args) { Tree tree = new Pine(); if( tree instanceof Pine ) System.out.println ("Pine"); else if( tree instanceof Tree ) System.out.println ("Tree"); else if( tree instanceof Oak ) System.out.println ( "Oak" ); else System.out.println ("Oops "); } }
Options- A. Pine
- B. Tree
- C. Forest
- D. Oops Discuss
Correct Answer: Pine
Explanation:
The program prints "Pine".
- 7. What will be the output of the program?
public class WrapTest { public static void main(String [] args) { int result = 0; short s = 42; Long x = new Long("42"); Long y = new Long(42); Short z = new Short("42"); Short x2 = new Short(s); Integer y2 = new Integer("42"); Integer z2 = new Integer(42); if (x == y) /* Line 13 */ result = 1; if (x.equals(y) ) /* Line 15 */ result = result + 10; if (x.equals(z) ) /* Line 17 */ result = result + 100; if (x.equals(x2) ) /* Line 19 */ result = result + 1000; if (x.equals(z2) ) /* Line 21 */ result = result + 10000; System.out.println("result = " + result); } }
Options- A. result = 1
- B. result = 10
- C. result = 11
- D. result = 11010 Discuss
Correct Answer: result = 10
Explanation:
Line 13 fails because == compares reference values, not object values. Line 15 succeeds because both String and primitive wrapper constructors resolve to the same value (except for the Character wrapper). Lines 17, 19, and 21 fail because the equals() method fails if the object classes being compared are different and not in the same tree hierarchy.
- 8. What will be the output of the program?
String x = "xyz"; x.toUpperCase(); /* Line 2 */ String y = x.replace('Y', 'y'); y = y + "abc"; System.out.println(y);
Options- A. abcXyZ
- B. abcxyz
- C. xyzabc
- D. XyZabc Discuss
Correct Answer: xyzabc
Explanation:
Line 2 creates a new String object with the value "XYZ", but this new object is immediately lost because there is no reference to it. Line 3 creates a new String object referenced by y. This new String object has the value "xyz" because there was no "Y" in the String object referred to by x. Line 4 creates a new String object, appends "abc" to the value "xyz", and refers y to the result.
- 9. What will be the output of the program?
public class StringRef { public static void main(String [] args) { String s1 = "abc"; String s2 = "def"; String s3 = s2; /* Line 7 */ s2 = "ghi"; System.out.println(s1 + s2 + s3); } }
Options- A. abcdefghi
- B. abcdefdef
- C. abcghidef
- D. abcghighi Discuss
Correct Answer: abcghidef
Explanation:
After line 7 executes, both s2 and s3 refer to a String object that contains the value "def". When line 8 executes, a new String object is created with the value "ghi", to which s2 refers. The reference variable s3 still refers to the (immutable) String object with the value "def".
- 10. What will be the output of the program?
int i = (int) Math.random();
Options- A. i = 0
- B. i = 1
- C. value of i is undetermined
- D. Statement causes a compile error Discuss
Correct Answer: i = 0
Explanation:
Math.random() returns a double value greater than or equal to 0 and less than 1. Its value is stored to an int but as this is a narrowing conversion, a cast is needed to tell the compiler that you are aware that there may be a loss of precision.
The value after the decimal point is lost when you cast a double to int and you are left with 0.
Programming
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