如何编写不影响其他集合的Set union方法？

The issue with your current union method is that it directly adds elements from the current set to the input set s, which modifies s permanently. A proper union operation should return a new set containing all elements from both original sets, leaving the original sets untouched.

Here's how to implement it correctly:

Step 1: Add Helper Methods to SetImp #

First, we need a way to access elements from another SetImp instance without modifying it. Add these public helper methods to your SetImp class:

// Returns the number of elements in the set
public int getCount() {
    return count;
}

// Returns the element at the specified index (throws exception if index is invalid)
public int getElement(int index) {
    if (index < 0 || index >= count) {
        throw new IndexOutOfBoundsException("Index out of range: " + index);
    }
    return set[index];
}

Step 2: Rewrite the Union Method #

Now, update the union method to create a new set, populate it with elements from both the current set and the input set, and return this new set:

public Set union(Set s){
    // Create a new SetImp instance to hold the union result
    SetImp unionSet = new SetImp();
    
    // Add all elements from the current set to the union set
    for(int i=0; i<count; i++){
        unionSet.add(set[i]);
    }
    
    // Add all elements from the input set s to the union set
    // Ensure s is an instance of SetImp before casting
    if (s instanceof SetImp) {
        SetImp sImp = (SetImp) s;
        for(int i=0; i<sImp.getCount(); i++){
            unionSet.add(sImp.getElement(i));
        }
    }
    
    return unionSet;
}

Why This Works #

• No modification to original sets: We create a brand new SetImp instance for the union result, so neither the current set nor the input set s are altered.
• Proper union logic: The new set includes all unique elements from both original sets (thanks to the add method checking for duplicates via contains).

Example Usage #

public static void main(String[] args) {
    SetImp set1 = new SetImp();
    set1.add(1);
    set1.add(2);
    
    SetImp set2 = new SetImp();
    set2.add(2);
    set2.add(3);
    
    Set unionResult = set1.union(set2);
    
    set1.display(); // Output: 1 2 (unchanged)
    set2.display(); // Output: 2 3 (unchanged)
    unionResult.display(); // Output: 1 2 3 (correct union)
}

内容的提问来源于stack exchange，提问作者ChingPoi