Java Lambda expression

A lambda expression in java is an implementation of a functional interface.
A functional interface is an interface which has ONLY 1 abstract method. It can have any number of default or static methods.
Lambda expression parameters
A lambda expression can have 1 or more than one parameters, if the functional interface method that it implements accepts parameters. Example,

public interface Output {

  public void print(String x);

}

In above code Output is a functional interface having only 1 abstract method print(), which expects 1 parameter.
Below is its implementation as a lambda expression

s -> System.out.println(s);

Notice that the parenthesis around parameter is not required when there is a single one.
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A lambda expression can have more than one and any number of parameters.
The number of parameters depends on the signature of interface method that it implements. Example,

public interface Operation {

  public int add(int x, int y);

}

Above interface abstract method expects 2 parameters.
Below is its lambda expression implementation

(a, b) -> return a + b;

Note that if there are multiple parameters in a lambda expression, then,
1. they must be enclosed between parenthesis.
2. they must be separated with a comma.
3. data types of those parameters are not required since they are inferred from the interface method declaration.

Following these rules, we can pass multiple parameters in a lambda expression in java 8.