How about:

    String input = "TestInput";

    StringBuilder b = new StringBuilder();
    b.append(input.charAt(0));
    for (int i = 1; i < input.length(); i++) {
        b.append("*").append(input.charAt(i));
    }
    System.out.println(b);
}

gives:

T*e*s*t*I*n*p*u*t

Is this what you wanted?

Edit: Pshemo's suggestion - use StringJoiner (Java 8 solution)

StringJoiner sj = new StringJoiner("*");
        for (int i = 0; i < input.length(); i++) {
            sj.add(input.substring(i, i + 1));
        }
        System.out.println(sj.toString());

And no StringBuilder version:

  String input="TestInput";
    String y = "";
    for (int i = 0; i < input.length(); i++) {
        y += "*" + input.charAt(i);
    }

And to your knowlage - using string concatenation is discouraged. It is better to use StringBuilder or StringJoiner

You could do this in a number of ways, for example...

You could attack the problem head on, for example...

StringBuilder sb = new StringBuilder(truck);
for (int index = 0; index < length; index++) {
    sb.append(0, " ");
}
truck = sb.toString();

You could write a method which produced an "empty" String

public String createPath(int length) {
    StringBuilder sb = new StringBuilder(length);
    for (int index = 0; index < length; index++) {
        sb.append(" ");
    }
    return sb.toString();
}

Then simply append the result, for example...

truck = createPath(n) + truck;

Or you could be really tricky and simply use String.format...

public String createPath(int length) {
    return String.format("%" + length + "s", "");
}

Add a loop to print z spaces before each character at z. Something like,

String str = "Compile";
for (int z = 0; z < str.length(); z++) {
    char ans = str.charAt(z);
    for (int x = 0; x < z; x++) {
        System.out.print(" ");
    }
    System.out.println(ans);
}

When you change s in the inner loop, you're just re-assigning the variable s, not the value stored in your linked list. Instead, you should loop through all the elements in your list updating them one by one. I think something like this should work:

//use recursion to add paths from all children to the list
for (TreeNode t : childList){
    paths.addAll(t.findStrings());
    //add nodevalue to the beginning of all strings in the list
    int length = paths.size();
    for (int i=0; i<length; i++) {
        paths.offer(nodevalue + paths.poll());
    }
}

poll takes the first item from the front of the list, and offer puts the result on the back. You take the first item off, change it, then put it on the back—repeat paths.size() times and you end up with the updated items in the original order.

1. String otherString = "helen" + character;

2. otherString +=  character;

The easiest way to for-each every char in a String is to use toCharArray():

for (char ch: "xyz".toCharArray()) {
}

This gives you the conciseness of for-each construct, but unfortunately String (which is immutable) must perform a defensive copy to generate the char[] (which is mutable), so there is some cost penalty.

From the documentation:

[toCharArray() returns] a newly allocated character array whose length is the length of this string and whose contents are initialized to contain the character sequence represented by this string.

There are more verbose ways of iterating over characters in an array (regular for loop, CharacterIterator, etc) but if you're willing to pay the cost toCharArray() for-each is the most concise.