Override Hashcode and Equals

18th February, 2023

The correct way to override equals and hashCode methods for Java classes, these should always be overridden together.

While this is a fun exercise and a good learning opportunity to implement this yourself it might be better in practice to use a library like Apache Commons Lang¹, which provides a EqualsBuilder and HashCodeBuilder or Java 7+‘s² Objects.hash and Objects.equals to simplify the process.

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public class Stock {
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    private String symbol;
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    private String exchange;
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    private long lotSize;
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    private int tickSize;
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    private boolean isRestricted;
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    private Date settlementDate;
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    private BigDecimal price;
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    @Override
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    public int hashCode() {
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        final int prime = 31;
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        int result = 1;
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        result = prime * result + ((exchange == null) ? 0 : exchange.hashCode());
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        result = prime * result + (isRestricted ? 1231 : 1237);
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        result = prime * result + (int) (lotSize ^ (lotSize >>> 32));
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        result = prime * result + ((price == null) ? 0 : price.hashCode());
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        result = prime * result + ((settlementDate == null) ? 0 : settlementDate.hashCode());
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        result = prime * result + ((symbol == null) ? 0 : symbol.hashCode());
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        result = prime * result + tickSize;
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        return result;
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    }
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    @Override
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    public boolean equals(Object obj) {
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        if (this == obj) return true;
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        if (obj == null || this.getClass() != obj.getClass()){
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            return false;
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        }
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        Stock other = (Stock) obj;
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        return
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            this.tickSize == other.tickSize &&
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            this.lotSize == other.lotSize &&
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            this.isRestricted == other.isRestricted &&
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            (this.symbol == other.symbol || (this.symbol != null && this.symbol.equals(other.symbol))) &&
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            (this.exchange == other.exchange|| (this.exchange != null && this.exchange.equals(other.exchange))) &&
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            (this.settlementDate == other.settlementDate|| (this.settlementDate != null && this.settlementDate.equals(other.settlementDate))) &&
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            (this.price == other.price|| (this.price != null && this.price.equals(other.price)));
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    }
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}

There are a few rules that the equals method must adhere to:

Reflexivity: For any non-null reference value x, x.equals(x) should return true.
Symmetry: For any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
Transitivity: For any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
Consistency: For any non-null reference values x and y, multiple invocations of x.equals(y) should consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
Non-nullity: For any non-null reference value x, x.equals(null) should return false.

Apache Commons Lang javadoc ↩
Java 7+ javadoc ↩

Override Hashcode and Equals

Footnotes