Data & Data Types

Important things to know

Constants

final int someConstant = 10;

Data Types

  • These are Java's primitive datatypes

  • you can use + operator on datatypes smaller than int otherwise anything that gets promoted to Integer.

  • To get max and min value of data type : 

int high = Integer.MAX_VALUE;
int low = Integer.MIN_VALUE;

Strings

Important functions

  • equalsIgnoreCase

  • toLowerCase

  • To check Strings in Java don't use "==" instead use :

    • str1.equals(str2)

  • charAt(0)

  • String.contains()

  • s.indexOf('i')

  • split(";")

  • join()

  • toString() Dont use with strings

  • String is a final and immutable class

  • To create a mutable string in java, Java has two classes

    • StringBuffer and

    • StringBuilder

String s = "this is an example";
String a = s.substring(11); 
// a will hold the string starting at character 11 
// until the end ("example")
String s = "popcorn";
System.out.println(s.replace('p','W'));

String Builder

StringBuilder sb = new StringBuilder();

sb.append("One=").append(one).append(", Color=red")

System.out.print(sb);

// Prints "One=1, Colour=red" 
// followed by an ASCII newline.
String result = sb.toString();

Formatter

int one = 1;
String color = "red";
Formatter f = new Formatter();
System.out.print(f.format("One=%d, colour=%s%n",
        one, color));
// Prints "One=1, Colour=red" 
// followed by the platform's line separator

String Joiner

StringJoiner sj = new StringJoiner(", ", "[", "]");
for (String s : new String[]{"A", "B", "C"}) {
    sj.add(s);
}
System.out.println(sj);
// Prints "[A, B, C]"

String Tokenizer

import java.util.StringTokenizer;
    public class Simple{
    public static void main(String args[]){
        StringTokenizer st = 
            new StringTokenizer("apple ball cat dog"," ");
        while (st.hasMoreTokens()) {
            System.out.println(st.nextToken());
        }
    }
}

Strings on Heap & Pool

  • created on heap, even literals

Difference between string on heap and Pool

String intern()

public class InternExample{  
    public static void main(String args[]){  
        String s1=new String("hello");  
        String s2="hello";  
        String s3=s1.intern();
        // returns string from pool, 
        // now it will be same as s2  
        System.out.println(s1==s2);
        // false because reference variables are 
        // pointing to different instance  
        System.out.println(s2==s3);
        // true because reference variables are 
        // pointing to same instance  
    }
}

Big Decimal

imports :

import java.math.BigDecimal;
import java.math.BigInteger;

Big Decimal objects are immutable

BigDecimal a = new BigDecimal("42.23");
BigDecimal b = new BigDecimal("10.001");
a.add(b); // a will still be 42.23
BigDecimal c = a.add(b); // c will be 52.231

Creating Big Decimals

BigDecimal a = new BigDecimal(5);

Comparing

a.compareTo(new BigDecimal(0));
// a is greater, returns 1
a.compareTo(new BigDecimal(5));
// a is equal, returns 0
a.compareTo(new BigDecimal(10));
// a is less, returns -1

Operations

Addition: add() method
Subtraction: subtract() method
Multiplication: multiply() method
Division: divide() method
BigDecimal a = new BigDecimal("5");
BigDecimal b = new BigDecimal("7");

//Equivalent to result = a + b
BigDecimal result = a.add(b);

//Equivalent to result = a - b
BigDecimal result = a.subtract(b);

//Equivalent to result = a * b
BigDecimal result = a.multiply(b);

//Equivalent to result = a / b
BigDecimal result = a.divide(b);

//Equivalent to result = a % b
BigDecimal result = a.remainder(b);

//Equivalent to result = a^10
BigDecimal result = a.pow(10);

//Equivalent to result = MAX(a,b)
BigDecimal result = a.max(b);

//Equivalent to result = MIN(a,b)
BigDecimal result = a.min(b);

BigDecimal a = new BigDecimal("5234.49843776");
// Moves the decimal point to 2 places left of 
// current position
BigDecimal result = a.movePointLeft(2);
// Result : 52.3449843776

BigDecimal a = new BigDecimal("5234.49843776");
// Moves the decimal point to 3 places right of 
// current position
BigDecimal result = a.movePointRight(3);
System.out.println(result);
// Result : 5234498.43776

String.Format

BigDecimal accountBalance = new BigDecimal("10000.00");
System.out.println(String.format("Account balance : %f",
accountBalance));

Best Practises

//Bad example:
BigDecimal bad0 = new BigDecimal(0);
BigDecimal bad1 = new BigDecimal(1);
BigDecimal bad10 = new BigDecimal(10);

//Good Example:
BigDecimal good0 = BigDecimal.ZERO;
BigDecimal good1 = BigDecimal.ONE;
BigDecimal good10 = BigDecimal.TEN;

Big Integer

Initialisation

BigInteger value1 = new BigInteger("10");
BigInteger value2 = new BigInteger("10");

  • To convert long or int values to Big Integer use:

long longValue = Long.MAX_VALUE;
BigInteger valueFromLong = BigInteger.valueOf(longValue);

  • To convert a numeric String to Big Integer use:

String decimalString = "-1";
BigInteger valueFromDecimalString = 
    new BigInteger(decimalString);

  • There are predefined constants for common values:

  • BigInteger.ZERO — value of "0".

  • BigInteger.ONE — value of "1".

  • BigInteger.TEN — value of "10".

Operations

BigInteger value1 = new BigInteger("10");
BigInteger value2 = new BigInteger("10");

BigInteger sum = value1.add(value2);
BigInteger sub = value1.subtract(value2);
BigInteger div = value1.divide(value2);
BigInteger mul = value1.multiply(value2);

BigInteger value1 = new BigInteger("10");
BigInteger power = value1.pow(3);

BigInteger power = value1.remainder(value2);

System.out.println(value1.gcd(value2));
System.out.println(value1.max(value2));

System.out.println(value1.min(value2));

Comparing

BigInteger one = BigInteger.valueOf(1);
BigInteger two = BigInteger.valueOf(2);

if(one.equals(two)){
    System.out.println("Equal");
}
else{
    System.out.println("Not Equal");
}

In general, do not use use the == operator to compare Big Integers

  • == operator: compares references; i.e. whether two values refer to the same object

  • equals() method: compares the content of two Big Integers.

BigInteger reallyBig = BigInteger.valueOf(10);
BigInteger reallyBig1 = BigInteger.valueOf(100);

if(reallyBig.compareTo(reallyBig1) == 0){
    //code when both are equal.
}
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