week01 Intro Flashcards
Explain about class in Java
class is a blueprint that defines a type, behaviour, and attribute
What is behaviour of class in Java?
what an object of a class can do
What is attribute of class in Java?
variables for an object
What does instantiate mean in Java?
to create an instance(object) of a class
Explain how Java works
.java file are compiled to .class file containing Java Bytecode. Java Bytecode is executed just-in-time by Java Virtual Machine(JVM) , then JVM interprets the Bytecode.
What letter case is used for class names in Java?
TitleCase
What letter case is used for variables and methods in Java?
camelCase
Why is Java strongly-typed?
because every ‘thing’ has a type
Expalin why Java is statically-typed
Variables in Java need a declared type, and a variable can store a single data type.
What is an identifier in other word?
a variable
What does initialization mean in Java?
to assign a value to a variable
What are the special points of primitive in Java?
- no special object created on the heap. the value is stored where it is used
- primitive can only store data within a specified range
- immutable.
What size is byte?
8 bits(integers in the range[-128, 127])
List 8 primitives
- byte
- short
- int
- long
- float
- double
- char
- boolean
What is the order of widening conversion in Java?
byte -> short -> int -> long -> float -> double
char -> int -> long -> float -> double
We may lose precision when converting from what to what? (widening)
when converting from a long to a float
What is the order of narrowing conversion in Java?
double -> float -> long -> int -> short -> byte/char
char -> byte/short
When we do narrowing conversion, range is preserved. true or false
false. range is not preserved. when widening, range is preserved.
When we do narrowing conversion, we may lose what? Answer all.
precision and magnitude
when widening, we may lose precision only.
How many operators in Java?
38
A Java variable whose data type is a primitive stores …
a value
A Java variable whose data type is a class stores …
an address
“Primitive can store null”
true or false?
false. we can’t assign null to primitive types.
we can pass null as a parameter to a method that expects an object reference.
new operator is used for what?
to instantiate a class(to create an object)
Why is String a special class?
because we can instantiate with a literal
例：
String greeting = “Hi”;
What does cross-platform distribution mean?
we can execute Java bytecode on any system as long as if you have JVM on your system
List 5 details for JVM
- cross-platform distribution
- manages and optimizes memory
- executes the program’s bytecodes via interpretation and just-in-time compilation
- Bytecode that is executed frequently is optimized and compiled directly into machine code
- performs garbage collection
What is identity in Java?
Identity is about the unique address of an object in memory. If two references point to the same address, they are identical(==)
What is equality in Java?
Equality is about the state or content of an object. Two objects are equal if they have the same content(.equals() method)
Is anotherNumber == number true or false?
Integer number = new Integer(2522);Integer anotherNumber = number;
True. anotherNumber and number point to the same object(address).
Is number.equals(thirdNumber)
true or false?
Integer number = new Integer(2522);
Integer thirdNumber = new Integer(2522);
True. number and thirdNumber have the same content even though they are different objects.
What is the main difference between == and .equals() in Java?
== checks if two references point to the same object. .equals() checks if two objects have the same content.
What does pass by value mean in Java method call?
- if the argument is a primitive, a copy of the value is passed to the method.
- if the argument is an object, a copy of the reference to the object is passed.
As a result, the original value or the object that the reference points to is not affected by what happens within the method.
What is String interning in Java?
String interning is Java’s process of reusing instances of strings that are identical, storing them in the String Constant Pool.
What do references to interned string point to?(How do variables connect to a string that’s been interned in Java’s memory?)
In Java, when you create strings that are exactly the same, Java doesn’t make new ones each time. Instead, all those strings are just pointers to one single string saved in a special place called the String Constant Pool.
How can you force string interning?
Use the .intern()method on a string to store it in the String Constant Pool.
Why is String interning important when comparing strings?
Because of interning, == compares memory addresses and may falsely imply string inequality. Always use .equals() for content comparison.
What are Java collections?
Java collections are data structures that hold objects. They use generic types, meaning you must use object types(no primitive types).
What does a Python List becomes in Java?
ArrayList<>
What does a Python Dictionary becomes in Java?
HashMap<K, V>
What does a Python Set become in Java?
HashSet<>
Can Java collections store primitive types directly?
No. Java collections cannot store primitive types like int, double. Use wrapper classes like Integer, Double.
What are wrapper classes in Java?
Wrapper classes turn primitive data types like int, char into object like Integer, Character
How do you create a wrapper object from a primitive?
Use the wrapper class constructor:
Integer wrappedInteger = new Integer(64738);Character wrappedChar = new Character('c');
How do you get the primitive value from a wrapper object?
Call methods like .intValue() or .charValue():
int value = wrappedInteger.intValue();char character = wrappedChar.charValue();
What is autoboxing?
Autoboxing is when Java automatically converts a primitive to a wrapper class when needed:
ArrayList<Integer> list = new ArrayList<>();list.add(5); // 5 is autoboxed to Integer
What is unboxing?
Unboxing is when Java automatically converts a wrapper class back to a primitive:
int num = list.get(0); // Integer is unboxed to int
How is getting input in Java different from Python?
Java uses the Scanner class for input, not global functions like Python. Java requires explicit import and object creation to read different data types.
How do you prepare to use Scanner in Java?
First, import it with import java.util.Scanner;. Then create a Scanner object like Scanner scan = new Scanner(System. in);
How does Scanner read different types of input in Java?
Use scan.nextInt() for integers, scan.nextDouble() for doubles, scan.next() for a single word, and scan.nextLine() for a line of text.
What is a common issue with Scanner in Java?
nextInt() and nextDouble() do not consume the newline character. nextLine()afterwards may read an empty line.
How do you handle the newline character after nextInt() in Java?
Before reading the next line of text, use scan.nextLine() to clear the newline from the buffer.
How do you compile Java code in the command line?
Use javac ClassName.java.
It compiles ClassName.java to ClassName.class.
How do you execute a compiled Java program in the command line?
Use java ClassName. Do not add .class after the class name.
What is the signature of the main method in Java?
public static void main(String[] args).
It accepts an array of String.
How do you pass command line arguments to a Java program?
Write them after the class name. Example:java Program arg1 arg2 arg3
What does a Java source file contain?
A .java file contains a single class definition written by the developer.
What does a Java class file contain?
A .class file contains the Java bytecode which is shareable, portable, and generated by the compiler from the .java file.
What does the Java compiler do?
It converts the .java file into a .class file that the Java interpreter can execute.
How does the Java Virtual Machine(JVM) execute Java bytecode?
During execution, the JVM interprets the .class file line by line and converts it to instructions that the CPU can understand.
What does the System class in Java provide?
Access to system resources including standard input(System.in), standard output(Sytem.out), error output streams(System.err), abd methods for loading files and libraries.
What is println in Java and which class does it belong to?
println is a method provided by the PrintStream class used to print a line of text to the current output stream.
What letter case do we use for constants?
All capital with “_” between words like UPPER_CASE.
What is the switch statement in Java.
The switch statement allows you to choose from a fixed number of options based on the value of a variable, which is more efficient than multiple if-else statements for this purpose.
What is the difference the while loop and do-while loop in Java?
The while loop checks its condition at the beginning before executing the loop’s body, whereas the do-while loop executes the body first and then checks the condition.
True or false?
“The do-while loop in Java will always executes its statements at least once.”
True. do-while loop is designed to execute the loop block once before checking the guard condition for the first time
