In addition to all of the changes that you’ve made, I would recommend:

1. ‘final’ class — already mentioned by Jesse and Lance
2. implements Serializable or perhaps Externalizable
3. implements Comparable — probably not needed by this class
4. implements Cloneable — probably not needed by this class
5. override toString() — not absolutely needed but always nice
6. scrap the getters and make the fields ‘public final’.

If the class is something that might need to be extended by a decorator in the future, item 6 is changed to:
6. define the getters in a separate interface that is implemented by this class.

public static int hashCode(Object a[]) {
if (a == null)
return 0;

int result = 1;

for (Object element : a)
result = 31 * result + (element == null ? 0 : element.hashCode());

return result;
}

Is this a better implementation than commons lang?

import com.google.common.base.Objects;

public class CustomerKey4 {
private final int customerId;
private final int districtId;

public CustomerKey4(final int customerId, final int districtId) {
this.customerId = customerId;
this.districtId = districtId;
}

public int getCustomerId() {
return customerId;
}

public int getDistrictId() {
return districtId;
}

@Override
public boolean equals(final Object obj) {
if (obj instanceof CustomerKey4) {
CustomerKey4 that = (CustomerKey4) obj;

return Objects.equal(customerId, that.customerId)
&& Objects.equal(districtId, that.districtId);
} else {
return false;
}
}

@Override
public int hashCode() {
return Objects.hashCode(customerId, districtId);
}
}

import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;

public class CustomerKey3 {
private final int customerId;
private final int districtId;

public CustomerKey3(final int customerId, final int districtId) {
this.customerId = customerId;
this.districtId = districtId;
}

public int getCustomerId() {
return customerId;
}

public int getDistrictId() {
return districtId;
}

@Override
public boolean equals(final Object obj) {
if (!(obj instanceof CustomerKey3)) {
return false;
}
if (this == obj) {
return true;
}
final CustomerKey3 rhs = (CustomerKey3) obj;
final EqualsBuilder builder = new EqualsBuilder();
builder.append(customerId, rhs.customerId);
builder.append(districtId, rhs.districtId);
return builder.isEquals();
}

@Override
public int hashCode() {
final HashCodeBuilder builder = new HashCodeBuilder(71, 499);
builder.append(customerId);
builder.append(districtId);
return builder.toHashCode();
}
}