Java Cheat Sheet and Common Int. Questions Flashcards
- Cover all of Java basic syntax and concepts - Cover common Java int. questions
What are all of the ways to compare a String in Java?
There are many ways to compare two Strings in Java:
Using == operator Using equals() method Using compareTo() method [Comparable interface compares values and returns an int, these int values may be less than, equal, or greater than 0] Using compareToIgnoreCase() method Using compare() method
Note: String is immutable in java. When two or more objects are created without new keyword, then both objects reference the same value. Double equals operator actually compares objects references, so should only be used for reference checking.
Thus, it’s highly preferable to use equals() or compareTo() when you want to check for value only for strings.
What is Reflection in Java? What are the most common Reflection methods?
Reflection
How do you convert an int[] or char[] to a String in Java?
How do you convert a String of numbers to int?
What about to an Integer?
What about Integer or int converted to a String?
//Primitive Array of int or char to a String:
int[] arr = new int[] {2,3,4,5};
String s = Arrays.toString(arr);
//String of numbers to int: String nums = "12345"; int i = Integer.parseInt(nums);
//String of numbers to Integer Integer j = Integer.valueOf(nums);
//Integer and int converted to a string int k = 234; Integer r = 567; String s = String.valueOf(k); String s2 = String.valueOf(r);
How do you create a List in Java WITHOUT using add to manually add in each value?
//Option 1:
List words = Arrays.asList("pen", "custom", "orphanage", "forest", "bubble", "butterfly");
//Option 2: List ints = new ArrayList(List.of(1,2,3));
Use Java streams to convert an ArrayList of Integer to a primitive int[], and then from primitive int[] to ArrayList
What about String? ArrayList of String to String[] and String[] to ArrayList of String? Without stream and with streams.
//Use stream to convert int[] to AL:
int[] ints = new int[] {1,2,3,4,5};
ArrayList ints_al = Arrays.stream(ints)
.boxed()
.collect(Collectors.toCollection(ArrayList::new));
//Convert from ArrayList to int[]
ArrayList al3 = new ArrayList(List.of(1,2,3));
int[] converted = al3.stream().mapToInt(v -> v).toArray();
//Convert ArrayList to String[]: no stream
ArrayList names = new ArrayList(List.of("Someone","You","Know"));
String[] prim_names = names.toArray(new String[names.size()]);
//Convert ArrayList to String[] using streams: String[] prim_names2 = names.stream().toArray(String[]::new); Arrays.stream(prim_names2).forEach(s2 -> System.out.print(s2 + " "));
//Convert String[] to ArrayList using stream:
String[] oldarr = new String[] {"Who","is","this"};
ArrayList newarr = Arrays.stream(oldarr).collect(Collectors.toCollection(ArrayList::new));
Convert an ArrayList of Integer to int[] using streams.
Bonus: How do you print a primitive array using streams?
//Use stream to convert ArrayList to int[]
ArrayList al2 = new ArrayList();
al2.add(6); al2.add(7); al2.add(8);
int[] prim_al2 = al2.stream().mapToInt(v -> v).toArray();
//Print primitive array using streams: Arrays.stream(prim_al2).forEach(i -> System.out.print(i + " "));
//Print array without using stream: System.out.println(Arrays.toString(prim_al2));
How do you print an int[] using forEach?
How do you print an ArrayList using forEach?
int[] numbs = new int[] {1,2,3,4,5};
Arrays.stream(numbs).forEach(i -> System.out.print(i + “ “));
ArrayList al3 = new ArrayList(List.of(5,4,3,2,1)); al3.forEach(j -> System.out.print(j))
Highest value from HashMap using streams?
Bonus: What about using a loop?
Answer
Highest key from a HashMap using streams?
Bonus: What about using a loop?
Answer
Key based on the highest value in a HashMap using streams
Bonus: Do it using a loop.
Answer
What are the three major ways to loop over an array in Java?
Bonus: What about an ArrayList ?
int[] nums = new int[10];
//Way #1: while loop
int i = 0;
while(i < nums.length) {
nums[i] = i;
i++;
}
//#2 for loop:
for(int j = 0; j < nums.length; j++)
{
nums[j] = j;
}
//Way #3: stream with forEach Arrays.stream(arr).forEach(x -> ...);
//Bonus: For an ArrayList, regular for loop using .size() as the limit, enhanced.for loop, or stream.
Palindrome
Answer
FizzBuzz
Answer
Reverse a linked list
Answer
How do you print the Java version and specification version to the console?
System.out.println(“Your Java version: “);
System.out.println(System.getProperty(“java.version”));
System.out.println(System.getProperty(“java.specification.version”));
How do you take input from the console in Java?
Scanner my_scan = new Scanner(System.in); String my_str = my_scan.nextLine();
What is the easiest way to reverse a String in Java?
String input = “something”;
String input = "Geeks for Geeks"; StringBuilder sb = new StringBuilder(); sb.append(input); sb.reverse(); String reversed = sb.toString(); System.out.println(reversed);
Convert a String to a StringBuilder and then to an int and an Integer:
//Convert String to StringBuilder to Integer String numbers ="123"; StringBuilder sb = new StringBuilder(numbers); int x = Integer.parseInt(sb.toString()); Integer y = Integer.valueOf(sb.toString()); System.out.println(x + '\n'); System.out.println(y + '\n');
How do you convert a String to a float in java?
Now, how do you round the float to 2 places?
What happens if the round does not need to be past two decimal places? How do you force the resultant String to have two decimal points? (Ie.4.99 -> 5.00)
import java.math.BigDecimal;
//Convert String to float: String N = "7132.1358"; float pf = Float.parseFloat(N); Float f = Float.valueOf(N);
//Round a float to certain decimal place: BigDecimal bd = new BigDecimal(N); bd = bd.setScale(2, BigDecimal.ROUND_HALF_EVEN); System.out.println(bd.floatValue());
//When round goes to .0 and does not need a second place, the above will not work once converted to a String. This code converts the BigDecimal to a String and then adds another 0:
String bds = bd.toString();
if(bds.substring(bds.length()-2,bds.length()).equals(".0"))
{
bds.concat("0");
}
