# Validate Credit Card Number in Java

## The challenge

Let’s implement the Luhn Algorithm, which is used to help validate credit card numbers.

Given a positive integer of up to 16 digits, return `true` if it is a valid credit card number, and `false` if it is not.

Here is the algorithm:

• Double every other digit, scanning from right to left, starting from the second digit (from the right).Another way to think about it is: if there are an even number of digits, double every other digit starting with the first; if there are an odd number of digits, double every other digit starting with the second:`1714 ==> [1*, 7, 1*, 4] ==> [2, 7, 2, 4] 12345 ==> [1, 2*, 3, 4*, 5] ==> [1, 4, 3, 8, 5] 891 ==> [8, 9*, 1] ==> [8, 18, 1]`
• If a resulting number is greater than `9`, replace it with the sum of its own digits (which is the same as subtracting `9` from it):`[8, 18*, 1] ==> [8, (1+8), 1] ==> [8, 9, 1] or: [8, 18*, 1] ==> [8, (18-9), 1] ==> [8, 9, 1]`
• Sum all of the final digits:`[8, 9, 1] ==> 8 + 9 + 1 = 18`
• Finally, take that sum and divide it by `10`. If the remainder equals zero, the original credit card number is valid.` 18 (modulus) 10 ==> 8 , which is not equal to 0, so this is not a valid credit card number`

## The solution in Java code

Option 1:

```.wp-block-code{border:0;padding:0}.wp-block-code>div{overflow:auto}.shcb-language{border:0;clip:rect(1px,1px,1px,1px);-webkit-clip-path:inset(50%);clip-path:inset(50%);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px;word-wrap:normal;word-break:normal}.hljs{box-sizing:border-box}.hljs.shcb-code-table{display:table;width:100%}.hljs.shcb-code-table>.shcb-loc{color:inherit;display:table-row;width:100%}.hljs.shcb-code-table .shcb-loc>span{display:table-cell}.wp-block-code code.hljs:not(.shcb-wrap-lines){white-space:pre}.wp-block-code code.hljs.shcb-wrap-lines{white-space:pre-wrap}.hljs.shcb-line-numbers{border-spacing:0;counter-reset:line}.hljs.shcb-line-numbers>.shcb-loc{counter-increment:line}.hljs.shcb-line-numbers .shcb-loc>span{padding-left:.75em}.hljs.shcb-line-numbers .shcb-loc::before{border-right:1px solid #ddd;content:counter(line);display:table-cell;padding:0 .75em;text-align:right;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none;white-space:nowrap;width:1%}```public class Validate {
public static boolean validate(String n) {
final boolean[] flag = {(n.length() & 1) == 1};
return Arrays.stream(
n.split(""))
.map(Integer::parseInt)
.mapToInt(value -> value)
.map(integer -> ((flag[0] ^= true) ? (integer * 2 - 1) % 9 + 1 : integer))
.sum() % 10 == 0;
}
}
```Code language: Java (java)```

Option 2:

``````public class Validate {
public static boolean validate(final String n){
if (n == null || n.isEmpty()) return false;
boolean x = true;
int sum = 0;
int temp = 0;

for (int i = n.length() - 1; i >= 0; i--) {
temp = n.charAt(i) - '0';
sum += (x = !x) ? temp > 4 ? temp * 2 - 9 : temp * 2 : temp;
}

return sum % 10 == 0;
}
}
```Code language: Java (java)```

Option 3:

``````public class Validate{
public static boolean validate(String n){
boolean[] flag = { (n.length() & 1) == 1 };
return n.chars()
.map(c -> c - '0')
.map(num -> (flag[0] ^= true) ? (num * 2 - 1) % 9 + 1 : num)
.sum() % 10 == 0;
}
}
```Code language: Java (java)```

## Test cases to validate our solution

``````import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class CreditcardValidationTest{
@Test
public void test891(){
assertEquals(false, Validate.validate("891"));
}
@Test
public void test123(){
assertEquals(false, Validate.validate("123"));
}
@Test
public void test1(){
assertEquals(false, Validate.validate("1"));
}
@Test
public void test2121(){
assertEquals(true, Validate.validate("2121"));
}
@Test
public void test1230(){
assertEquals(true, Validate.validate("1230"));
}
@Test
public void test8675309(){
assertEquals(false, Validate.validate("8675309"));
}
@Test
public void test4111111111111111(){
assertEquals(true, Validate.validate("4111111111111111"));
}
@Test
public void test26(){
assertEquals(true, Validate.validate("26"));
}
@Test
public void test2626262626262626(){
assertEquals(true, Validate.validate("2626262626262626"));
}
@Test
public void test91(){
assertEquals(true, Validate.validate("91"));
}
@Test
public void test92(){
assertEquals(false, Validate.validate("92"));
}
@Test
public void test912030(){
assertEquals(true, Validate.validate("912030"));
}
@Test
public void test922030(){
assertEquals(false, Validate.validate("922030"));
}
}
```Code language: Java (java)```

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