/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(combine("ABCD", "abcdef"));
		System.out.println(combine("ABCDEF", "abcd"));
		System.out.println(combine("ABCDEF", "abcdef"));
		System.out.println(combine("ABCD", null));
		System.out.println(combine(null, "abcd"));
		System.out.println(combine(null, null));
	}
 
	public static String combine(String str1,String str2) {
		StringBuilder result = new StringBuilder();
        int str1size = str1 == null ? 0 : str1.length();
        int str2size = str2 == null ? 0 : str2.length();
		int lowerSize = Math.min(str1size, str2size);
		int greaterSize = Math.max(str1size, str2size);
        int i;  // define the counter variable outside of a for, because we will reuse it in the following for loops
		for (i=0; i < lowerSize; i++) {                              // browse the common part of the strings
			result.append(str1.charAt(i)).append(str2.charAt(i));
		}
		for (int upTo = Math.min(greaterSize, str1size); i < upTo ; i++) {     // browse the remaining part of str1, if applicable
			result.append(str1.charAt(i));
		}
		for (int upTo = Math.min(greaterSize, str2size); i < upTo; i++) {     // browse the remaining part of str2, if applicable
			result.append(str2.charAt(i));
		}
		return result.toString();
	}
}

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