Java ArrayList replace at specific index

Pro tip: Avoiding index out of bounds

Programs hate surprises, like IndexOutOfBoundsException. Always check if the index is valid:

public void safeReplace(ArrayList<String> list, int index, String newElement) {
    // We're not the wild west. We check our guns ... eh values, I mean.
    if (index >= 0 && index < list.size()) {
        list.set(index, newElement);
    } else {
        throw new IndexOutOfBoundsException("Hold your horses! That index doesn't exist: " + index);
    }
}
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Find it first, then replace

Don’t know the index? No problem! Find it with indexOf, then replace it:

int indexToReplace = list.indexOf("Old");
if (indexToReplace != -1) {
    list.set(indexToReplace, "New"); // Old, you're getting a makeover!
}
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When conditions apply: conditional replacement

Need to replace only specific elements? Iterate and put condition in set:

for (int i = 0; i < list.size(); i++) {
    if ("Old".equals(list.get(i))) { // "Old", it’s not you, it’s me… 
        list.set(i, "New");
    }
}
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Advancing your skills: effective usage of set()

• Always access elements with get() prior to replacing them with set().
• set() only trades places. The list size stays the same.
• Working with custom objects? Make sure they override equals and hashCode for accurate indexOf operations.

Building your own stage: custom replace method

A custom replaceLightBulb method can be a handy tool:

public <T> void replaceLightBulb(ArrayList<T> list, int index, T newElement) {
    // Someone's gotta check the boundaries
    if (index < 0 || index >= list.size()) {
        throw new IndexOutOfBoundsException("Nope! Can't replace a bulb that doesn't exist!"); 
    }

    // Out with the old, in with the new!
    T oldElement = list.get(index);
    list.set(index, newElement);
    System.out.println("The old " + oldElement + " is out. Hello to " + newElement + " at index " + index);
}
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