Chapter 1 Building Blocks Flashcards
Review Questions
1. Which of the following are legal entry point methods that can be run from the command line? (Choose all that apply.)
A. private static void main(String[] args)
B. public static final main(String[] args)
C. public void main(String[] args)
D. public static final void main(String[] args)
E. public static void main(String[] args)
F. public static main(String[] args)
E
D, E.
- Option E is the canonical main() method signature. You need to memorize it.
- Option D is an alternate form with the redundant final.
- Option A is incorrect because the main() method must be public.
- Options B and F are incorrect because the main() method must have a void return type.
- Option C is incorrect because the main() method must be static.
2. Which answer options represent the order in which the following statements can be assembled into a program that will compile successfully? (Choose all that apply.)
X: class Rabbit {}
Y: import java.util.*;
Z: package animals;
A. X, Y, Z
B. Y, Z, X
C. Z, Y, X
D. Y, X
E. Z, X
F. X, Z
G. None of the above
C, D, E
C, D, E.
* The package and import statements are both optional.
* If both are present, the order must be package, then import, and then class.
* Option A is incorrect because class is before package and import.
* Option B is incorrect because import is before package. Option F is incorrect because class is before package.
3. Which of the following are true? (Choose all that apply.)
public class Bunny {
public static void main(String[] x) {
Bunny bun = new Bunny();
} }
A. Bunny is a class.
B. bun is a class.
C. main is a class.
D. Bunny is a reference to an object.
E. bun is a reference to an object.
F. main is a reference to an object.
G. The main() method doesn’t run because the parameter name is incorrect.
A, E
A, E.
* Bunny is a class, which can be seen from the declaration: public class Bunny.
* The variable bun is a reference to an object.
* The method main() is the standard entry point to a program.
* Option G is incorrect because the parameter type matters, not the parameter name.
4. Which of the following are valid Java identifiers? (Choose all that apply.)
A. _
B. _helloWorld$
C. true
D. java.lang
E. Public
F. 1980_s
G. _Q2_
B, E, G
B, E, G.
- Option A is invalid because a single underscore is not allowed.
- Option C is not a valid identifier because true is a Java reserved word.
- Option D is not valid because a period (.) is not allowed in identifiers.
- Option F is not valid because the
first character is not a letter, dollar sign ($), or underscore (_). - Options B, E, and G are valid because they contain only valid characters.
5. Which statements about the following program are correct? (Choose all that apply.)
2: public class Bear {
3: private Bear pandaBear;
4: private void roar(Bear b) {
5: System.out.println("Roar!");
6: pandaBear = b;
7: }
8: public static void main(String[] args) {
9: Bear brownBear = new Bear();
10: Bear polarBear = new Bear();
11: brownBear.roar(polarBear);
12: polarBear = null;
13: brownBear = null;
14: System.gc(); } }
A. The object created on line 9 is eligible for garbage collection after line 13.
B. The object created on line 9 is eligible for garbage collection after line 14.
C. The object created on line 10 is eligible for garbage collection after line 12.
D. The object created on line 10 is eligible for garbage collection after line 13.
E. Garbage collection is guaranteed to run.
F. Garbage collection might or might not run.
G. The code does not compile.
A, D, F
A, D, F.
- Garbage collection is never guaranteed to run, making option F correct and option E incorrect.
- Next, the class compiles and runs without issue, so option G is incorrect.
- The Bear object created on line 9 is accessible until line 13 via the brownBear reference variable, which is option A.
- The Bear object created on line 10 is accessible via both the polarBear reference and the brownBear.pandaBear reference.
- After line 12, the object is still accessible via brownBear.pandaBear.
- After line 13, though, it is no longer accessible since brownBear is no longer accessible, which makes option D the final answer.
6. Assuming the following class compiles, how many variables defined in the class or method are in scope on the line marked on line 14?
1: public class Camel {
2: { int hairs = 3_000_0; }
3: long water, air=2;
4: boolean twoHumps = true;
5: public void spit(float distance) {
6: var path = "";
7: { double teeth = 32 + distance++; }
8: while(water> 0) {
9: int age = twoHumps ? 1 : 2;
10: short i=-1;
11: for(i=0; i<10; i++) {
12: var Private = 2;
13: }
14: // SCOPE
15: }
16: }
17: }
A. 2
B. 3
C. 4
D. 5
E. 6
F. 7
G. None of the above
F
F.
- To solve this problem, you need to trace the braces {} and see when variables go in and out of scope.
- The variables on lines 2 and 7 are only in scope for a single line block.
- The variable on line 12 is only in scope for the for loop.
- None of these are in scope on line 14.
- By contrast, the three instance variables on lines 3 and 4 are available in all instance methods.
- Additionally, the variables on lines 6, 9, and 10 are available since the method and while loop are still in scope.
- This is a total of 7 variables, which is option F.
7. Which are true about this code? (Choose all that apply.)
public class KitchenSink {
private int numForks;
public static void main(String[] args) {
int numKnives;
System.out.print("""
"# forks = " + numForks +
" # knives = " + numKnives +
# cups = 0""");
}
}
A. The output includes: # forks = 0.
B. The output includes: # knives = 0.
C. The output includes: # cups = 0.
D. The output includes a blank line.
E. The output includes one or more lines that begin with whitespace.
F. The code does not compile.
F
C, E.
- The first thing to recognize is that this is a text block and the code inside the
“”” is just text. - Options A and B are incorrect because the numForks and numKnives variables are not used.
- This is convenient since numKnives is not initialized and would not compile if it were referenced.
- Option C is correct as it is matching text.
- Option D is incorrect because the text block does not have a trailing blank line.
- Finally, option E is also an answer since “ # knives is indented.
8. Which of the following code snippets about var compile without issue when used in a method? (Choose all that apply.)
A. var spring = null;
B. var fall = “leaves”;
C. var evening = 2; evening = null;
D. var night = Integer.valueOf(3);
E. var day = 1/0;
F. var winter = 12, cold;
G. var fall = 2, autumn = 2;
H. var morning = “”; morning = null;
B, D, E, H
B, D, E, H.
- A var cannot be initialized with a null value without a type, but it can be assigned a null value later if the underlying type is not a primitive.
- For these reasons, option H is correct, but options A and C are incorrect.
- Options B and D are correct as the underlying types are String and Integer, respectively.
- Option E is correct as this is a valid numeric expression.
- You might know that dividing by zero produces a runtime exception, but the question was only about whether the code compiled.
- Finally, options F and G are incorrect as var cannot be used in a multiple-variable assignment.
9. Which of the following are correct? (Choose all that apply.)
A. An instance variable of type float defaults to 0.
B. An instance variable of type char defaults to null.
C. A local variable of type double defaults to 0.0.
D. A local variable of type int defaults to null.
E. A class variable of type String defaults to null.
F. A class variable of type String defaults to the empty string “”.
G. None of the above.
E
E.
- Options C and D are incorrect because local variables don’t have default values.
- Option A is incorrect because float should have a decimal point.
- Option B is incorrect because primitives do not default to null.
- Option E is correct and option F incorrect because reference types in class variables default to null.
10. Which of the following expressions, when inserted independently into the blank line, allow the code to compile? (Choose all that apply.)
public void printMagicData() {
var magic = \_\_\_\_\_\_\_\_\_\_\_\_\_\_;
System.out.println(magic);
}
A. 3_1
B. 1_329_.0
C. 3_13.0_
D. 5_291._2
E. 2_234.0_0
F. 9\_\_\_6
G. _1_3_5_0
A, E, F
A, E, F.
- An underscore
(_)can be placed in any numeric literal, as long as it is not at the beginning, at the end, or next to a decimal point (.). - Underscores can even be placed next to each other.
- For these reasons, options A, E, and F are correct.
- Options B and D are incorrect as the underscore
(_)is next to a decimal point (.). - Options C and G are incorrect because an underscore
(_)cannot be placed at the beginning or end of the literal.
11. Given the following two class files, what is the maximum number of imports that can be removed and have the code still compile?
// Water.java
package aquarium;
public class Water { }
// Tank.java
package aquarium;
import java.lang.*;
import java.lang.System;
import aquarium.Water;
import aquarium.*;
public class Tank {
public void print(Water water) {
System.out.println(water); } }
A. 0
B. 1
C. 2
D. 3
E. 4
F. Does not compile
E
E.
- The first two imports can be removed because java.lang is automatically imported.
- The following two imports can be removed because Tank and Water are in the same package, making the correct option E.
- If Tank and Water were in different packages, exactly one of these two imports could be removed. In that case, the answer would be option D.
12. Which statements about the following class are correct? (Choose all that apply.)
1: public class ClownFish {
2: int gills = 0, double weight=2;
3: { int fins = gills; }
4: void print(int length = 3) {
5: System.out.println(gills);
6: System.out.println(weight);
7: System.out.println(fins);
8: System.out.println(length);
9: } }
A. Line 2 generates a compiler error.
B. Line 3 generates a compiler error.
C. Line 4 generates a compiler error.
D. Line 7 generates a compiler error.
E. The code prints 0.
F. The code prints 2.0.
G. The code prints 2.
H. The code prints 3.
A, C, D
A, C, D.
- Line 2 does not compile as only one type should be specified, making option A correct.
- Line 3 compiles without issue as it declares a local variable inside an instance initializer that is never used.
- Line 4 does not compile because Java does not support setting default method parameter values, making option C correct.
- Finally, line 7 does not compile because fins is in scope and accessible only inside the instance initializer on line 3, making option D correct.
13. Given the following classes, which of the following snippets can independently be inserted in place of INSERT IMPORTS HERE and have the code compile? (Choose all that apply.)
package aquarium;
public class Water {
boolean salty = false;
}
package aquarium.jellies;
public class Water {
boolean salty = true;
}
package employee;
INSERT IMPORTS HERE
public class WaterFiller {
Water water;
}
A. import aquarium.*;
B. import aquarium.Water;import aquarium.jellies.*;
C. import aquarium.*;
import aquarium.jellies.Water;
D. import aquarium.*;import aquarium.jellies.*;
E. import aquarium.Water;
import aquarium.jellies.Water;
F. None of these imports can make the code compile.
A, B, C, E
A, B, C.
- Option A is correct because it imports all the classes in the aquarium package including aquarium.Water.
- Options B and C are correct because they import Water by class name. Since importing by class name takes precedence over wildcards, these compile.
- Option D is incorrect because Java doesn’t know which of the two wildcard Water classes to use.
- Option E is incorrect because you cannot specify the same class name in two imports.
14. Which of the following statements about the code snippet are true? (Choose all that apply.)
3: short numPets = 5L; 4: int numGrains = 2.0; 5: String name = "Scruffy"; 6: int d = numPets.length(); 7: int e = numGrains.length; 8: int f = name.length();
A. Line 3 generates a compiler error.
B. Line 4 generates a compiler error.
C. Line 5 generates a compiler error.
D. Line 6 generates a compiler error.
E. Line 7 generates a compiler error.
F. Line 8 generates a compiler error.
A, B, D, E
A, B, D, E.
- Line 3 does not compile because the L suffix makes the literal value a long, which cannot be stored inside a short directly, making option A correct.
- Line 4 does not compile because int is an integral type, but 2.0 is a double literal value, making option B correct.
- Line 5 compiles without issue. Lines 6 and 7 do not compile because numPets and numGrains are both primitives, and you can call methods only on reference types, not primitive values, making options D and E correct, respectively.
- Finally, line 8 compiles because there is a length() method defined on String.
15. Which of the following statements about garbage collection are correct? (Choose all that apply.)
A. Calling System.gc() is guaranteed to free up memory by destroying objects eligible for garbage collection.
B. Garbage collection runs on a set schedule.
C. Garbage collection allows the JVM to reclaim memory for other objects.
D. Garbage collection runs when your program has used up half the available memory.
E. An object may be eligible for garbage collection but never removed from the heap.
F. An object is eligible for garbage collection once no references to it are accessible in the program.
G. Marking a variable final means its associated object will never be garbage collected.
C, E, F
C, E, F.
- In Java, there are no guarantees about when garbage collection will run.
- The JVM is free to ignore calls to System.gc(). For this reason, options A, B, and D
are incorrect. - Option C is correct as the purpose of garbage collection is to reclaim used memory.
- Option E is also correct that an object may never be garbage collected, such as if the program ends before garbage collection runs.
- Option F is correct and is the primary means by which garbage collection algorithms determine whether an object is eligible for garbage collection.
- Finally, option G is incorrect as marking a variable final means it is constant within its own scope.
- For example, a local variable marked final will be eligible for garbage collection after the method ends, assuming there are no other references to the object that exist outside the method.
