Removing duplicates without overriding hashCode() [duplicate]

Question

For enterprise reasons I can't override hashCode and I must use Java 6 (but I can use guava)

Whats the bests/simplest/quickest/most efficient/[insert indeterminate adjective equivalent to best] mechanism to remove duplicate beans from a Java collection?

A duplicate is defined by a subset of getters returning same values, e.g.

pojoA.getVal() == pojoB.getVal() && pojoA.getOtherVal() == pojoB.getOtherVal()

Answer 1

Wrap the objects of interest into your own class, and override its hashCode/equals to pay attention to a specific subset of attributes. Make a hash set of wrappers, then harvest the objects from the set to get a duplicate-free subset.

Here is an example:

class ActualData {
    public String getAttr1();
    public String getAttr2();
    public String getAttr3();
    public String getAttr4();
}

Let's say you want to pay attention to attributes 1, 2, and 4. Then you can make a wrapper like this:

class Wrapper {
    private final ActualData data;
    public ActualData getData() {
        return data;
    }
    private final int hash;
    public Wrapper(ActualData data) {
        this.data = data;
        this.has = ... // Compute hash based on data's attr1, 2, and 4
    }
    @Override
    public int hashCode() {
        return hashCode;
    }
    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof Wrapper)) return false;
        Wrapper other = (Wrapper)obj;
        return data.getAttr1().equals(other.getAttr1())
            && data.getAttr2().equals(other.getAttr2())
            && data.getAttr4().equals(other.getAttr4());
    }
}

Now you can make a HashSet<Wrapper>:

Set<Wrapper> set = new HashSet<>();
for (ActualData item : listWithDuplicates) {
    if (!set.add(new Wrapper(item))) {
        System.out.println("Item "+item+" was a duplicate");
    }
}