Effective Java

---

ITEM 42: PREFER LAMBDAS TO ANONYMOUS CLASSES
195
// Enum type with constant-specific class bodies & data (Item 34)
public enum Operation {
PLUS("+") {
public double apply(double x, double y) { return x + y; }
},
MINUS("-") {
public double apply(double x, double y) { return x - y; }
},
TIMES("*") {
public double apply(double x, double y) { return x * y; }
},
DIVIDE("/") {
public double apply(double x, double y) { return x / y; }
};
private final String symbol;
Operation(String symbol) { this.symbol = symbol; }
@Override public String toString() { return symbol; }
public abstract double apply(double x, double y);
}
Item 34 says that enum instance fields are preferable to constant-specific class
bodies. Lambdas make it easy to implement constant-specific behavior using the
former instead of the latter. Merely pass a lambda implementing each enum
constant’s behavior to its constructor. The constructor stores the lambda in an
instance field, and the 
apply
method forwards invocations to the lambda. The
resulting code is simpler and clearer than the original version: