Comp Sci Midterm – Flashcards
90 test answers
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Every statement in Java must end with a
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semicolon
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What is an object oriented programming language?
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c++, Java, C#
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Java source code can be executed on a Java virtual Machine
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false
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If a program compiles fine, but it produces an incorrect result then the program suffers,
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a logic error.
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9 Suppose you define a Java class as follows: public class Test { } In order to compile this program, the source code should be stored in a file named
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Test.java
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What method displays a message in a dialog box?
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A. JOptionPane.showMessageDialog(null, "Welcome to Java!")? C. JOptionPane.showMessageDialog(null, "Welcome to Java!", "Example 1.2 Output", JOptionPane.INFORMATION_MESSAGE)?
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How do you correctly display We correctly display, "Welcome to Java!"
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B. System.out.print("Welcome to Java")? C. System.out.println("Welcome to Java")?
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What loads the Java bytecode to the memory?
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class loader
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The JDK command to compile a class in the file Test.java is
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javac Test.java
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Who developed Java
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Sun Microsystem
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Java compiler translates Java source code into
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Java bytecode
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What is Architecture Neutral?
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Java
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Java virtual Machine
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is a software that interprets Java bytecode
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You can use
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java, to run a java program
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A compiler
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translates high level language program into machine language program
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.java
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the extension name of a java source code file
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softwares and programs
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are instructions to the computer
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the speed of the cpu may be measured in
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gigahertz/megahertz
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programming style is important because
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good programming makes a program more readable. and a good programming style helps reduce errors.
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a block is enclosed inside
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braces
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25 To use JOptionPane in your program, you may import it using:
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A. import javax.swing.*? D. import javax.swing.JOptionPane?
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What does line 1 define in a program?
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A class, Every Java program must have at least one class. Each class have a name. By convention, class names start with an uppercase letter.
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What does line 2 define in a program?
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The main method. The program is executed from the main method. A class may contain several methods. The main method is the entry point where the program begins execution.
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What does a method contain?
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It is constructed of statements. The main method is this program contains the System.out.println statement. This statement displays the string Welcome to Java! on the console (line4). String is a programming term meaning a sequence of characters/ A string must be enclosed in double quotation marks. Every statement in Java ends with a semicolon; known as a statement terminator.
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What are reserved words or keywords?
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they have a specific meaning to the compiler and cannot be used for other purposes in the program. For example, when the compiler sees the word class, it understands that the word after class is the name fir the class. Other reserved words in this program, are public, static, and void.
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What is Line 3?
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It is a comment that documents what the program is and how it is constructed. Comments help programmers to communicate and understand the program. They are not programming statements and thus are ignored by the compiler. In java, comments are preceeded by 2 //, on a line, called a line comment. or ecnclosed /* and */ on one or several lines, called a block comment or paragraph comment. When the compiler sees //, it ignores all text on the same line. When it sees /*, it scans for the next */ and ignores any text.
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What do a pair of curly braces form?
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The form a block that groups the program's components. In Java, each block begins with a opening brace {, and ends with a closing }, Every class has a class block that groups the data and methods of the class. Similarly every method has a method block that groups the statement in the method. Blocks can be nested, meaning that one block can be placed within another.
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Syntax errors,
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most common error. ( if a semicolon is missing, a brace is missing, a quotation mark is missing, or a word is misspelled) {}- denotes a block to enclose statements ()= used with methods [] denote an array // precede a comment line "" enclose a string ( sequence of characters) ; marks the end of a statement
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What do you do if your program has run time errors?
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you have to modify it, recompile it, and execute it again.You can use any text editor or IDE to create or edit Java source code file.
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What happens if you do not have any syntax errors?
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The compiler generates a bytecode file with a .class extension. This, the preceding commanding generates a file named Welcome.class. Java is translated into byte code and can be executed by any program or machine.
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The java language
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is a high level language, but java bytecode is a low level language. The bytecode is similar to machine instructions, but is architecture neutral and can run on any platform that has Java virtual Machine. Java bytecode can run on a variety of hardware platforms and operating systems. Java source code is compiled into java bytecode and java bytecode is interpreted by JVM.
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javac
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Welcome.java
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Welcome.class
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java command
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When executing a program,
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The java virtual machine first loads the bytecode of the class memory using a program classed the class order. If your pgroam uses other classes, the class loader dynamically loads them just before they are needed. After a class, is loader, the JVM uses a program called the bytecode verifer to check the validity of the bytecode and to ensure that the bytecode does not violate Java's security restrictions. Java enforces strict security to make sure files are not tampered with.
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To rewrite a text in a message box you use:
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The Joption class option! it is one of the many predefine classed in the Java library. you can use this rather than reinvent the wheel.
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There are two types of block statements
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next line style, and end of line style. The next line style aligns the braces vertically and makes programs easy to read, whereas the end of line style saves space, and avoids errors.
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What are runtime errors
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They are errors that cause a program to terminate abnormally. They occur while a program is running in an environment that detects an operation that is impossible to carry out. Input mistakes typically cause runtime errors. An input erros occurs when the program is waiting for the user to enter a value, but the user enters a value that the program cannot handle. For instance, if the program expects to read in a number, but instead the user enters a string, this causes data-type errors.
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Logic errors
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The program does not perform what it is intended to do
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compile java
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javac
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run java
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java command
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the keyword class introduces, a
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class definition, which is included in the block.
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Main methods are contained in a class.
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To run a java program, the program must have a main method. The main method is is the entry point where the program will start execution.
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Reserved words, or keywords
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have a specific meaning to the compiler, and cannot be used for other purposes.
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specific import
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specifies a single class to be imported. Example: import javax.swing.JOptionPane; (Ch. 1 p. 17)
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wildcard import
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imports all classes in a package. Example: import javax.swing.*; (Ch. 1 p. 17)
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What is the Unicode for the character A is 65. The expression 'A'+1 evaluates to
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B. 66
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5 % 2
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1
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Which of the following is a constant according to Java naming conventions?
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COUNT, MAX_VALUE
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What keyword must be declared to be used as a constant?
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final
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15 % 4 is _____
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B. 3
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What happens if you attempt to add an int, a byte, a long, and a double, the result will be a
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double value.
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public class Test { public static void main(String[] args) { int x = 1? int y = x++ + x? System.out.println("y is " + y)? } }
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y is three
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9 Which of the following assignment statements is incorrect?
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A. i == j == k == 1? D. i = 1 = j = 1 = k = 1?
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What statements are not illegal?
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A. short s = 10? C. int t = 23? D. float f = 34?
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To declare an int variable number with initial value 2, you write
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int number=2
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12 According to Java naming convention, which of the following names can be variables?
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C. totalLength A. findArea
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The assignment operator in Java is
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=
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What value is printed? public static void main(String[] args) { int j = 0? int i = ++j + j * 5? System.out.println("What is i? " + i)? } }
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6
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15 Analyze the following code. import javax.swing.*? public class ShowErrors { public static void main(String[] args) { int i? int j? String s = JOptionPane.showInputDialog(null, "Enter an integer", "Input", JOptionPane.QUESTION_MESSAGE)? j = Integer.parseInt(s)? i = (i + 4)? }
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D. The program cannot compile because i does not have an initial value when it is used in i = i + 4?
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Every letter in Java is lowercase
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true
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a variable may be assignment a value only once in a program
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false
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Which of the following statements are correct?
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A. char c = 100? C. char c = 'd'?
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A java statement ends with a semicolon
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true
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To declare a constant MAX_LENGTH inside a method with value 99.98, you write
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C. final double MAX_LENGTH = 99.98?
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the value of a variable can be changed
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true
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The __________ method displays an input dialog for reading a string.
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A. String string = JOptionPane.showInputDialog("Enter a string")? D. String string = JOptionPane.showInputDialog(null, "Enter a string", "Input Demo", JOptionPane.QUESTION_MESSAGE)? E. String string = JOptionPane.showInputDialog(null, "Enter a string")?
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The following code fragment reads in two numbers: Scanner input = new Scanner(System.in)? int i = input.nextInt()? double d = input.nextDouble()? What are the correct ways to enter these two numbers?
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A. Enter an integer, a space, a double value, and then the Enter key. B. Enter an integer, an Enter key, a double value, and then the Enter key. D. Enter an integer, two spaces, a double value, and then the Enter key.
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25 What is "Welcome" + 1 + 1*2?
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D. Welcome12
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A constant can be defined using using the final keyword.
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true
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Suppose x is a char variable with a value 'b'. What will be displayed by the statement System.out.println(++x)?
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c
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Are the following four statements equivalent? number += 1? number = number + 1? number++? ++number?
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yes
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Which of the following are not valid assignment statements?
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55=x;
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5 Which of the following assignment statements is correct to assign character 5 to c?
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B. char c = '5'?
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You can assign the value in ________ to an int variable.
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B. 93 C. 'x'
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What is x after the following statements? int x = 2? int y = 1? x *= y + 1?
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D. x is 4.
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8 Which of the following are the same as 1545.534?
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A. 1545534.0e3 B. 154553.4e2 C. 1.545534e+3 D. 0.1545534e+4
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The expression 4 + 20 / (3- 1) * 2 is evaluated to
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24
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To declare a constant PI, you write
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D. final double PI = 3.14159?
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The __________ method parses a string s to a double value.
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C. Double.parseDouble(s)?
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12 Which of the following are correct names for variables according to Java naming conventions?
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B. radius C. findArea
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-15 % 4 is _____
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D. -3
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You can cast a character value to an int or an int to a char
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true
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15 A variable may be assigned a value only once in the program.
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false
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17 What is the exact output of the following code? double area = 3.5? System.out.print("area")? System.out.print(area)?
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area3.5
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18 What is the value of (double)5/2?
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2.5
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If you attempt to add an int, a byte, a long, and a double, the result will be a __________ value.
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double
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