java how to make a parameter optional

1void foo(String a, Integer b) {
2    //...
3}
4
5void foo(String a) {
6    foo(a, 0); // here, 0 is a default value for b
7}
8
9foo("a", 2);
10foo("a");

1void foo(String a, Optional<Integer> bOpt) {
2    Integer b = bOpt.isPresent() ? bOpt.get() : 0;
3    //...
4}
5
6foo("a", Optional.of(2));
7foo("a", Optional.<Integer>absent());

1void foo(String a, Integer... b) {
2    Integer b1 = b.length > 0 ? b[0] : 0;
3    Integer b2 = b.length > 1 ? b[1] : 0;
4    //...
5}
6
7foo("a");
8foo("a", 1, 2);

1void foo(String a, Object... b) {
2    Integer b1 = 0;
3    String b2 = "";
4    if (b.length > 0) {
5      if (!(b[0] instanceof Integer)) { 
6          throw new IllegalArgumentException("...");
7      }
8      b1 = (Integer)b[0];
9    }
10    if (b.length > 1) {
11        if (!(b[1] instanceof String)) { 
12            throw new IllegalArgumentException("...");
13        }
14        b2 = (String)b[1];
15        //...
16    }
17    //...
18}
19
20foo("a");
21foo("a", 1);
22foo("a", 1, "b2");

1//Java 9 and above only    
2@SuppressWarnings("unchecked")
3    static <T> T getParm(Map<String, Object> map, String key, T defaultValue)
4    {
5        return (map.containsKey(key)) ? (T) map.get(key) : defaultValue;
6    }
7
8    void foo(Map<String, Object> parameters) {
9        String a = getParm(parameters, "a", "");
10        int b = getParm(parameters, "b", 0);
11        // d = ...
12    }
13
14    foo(Map.of("a","a",  "b",2,  "d","value"));

1 class Foo {
2     private final String a; 
3     private final Integer b;
4
5     Foo(String a, Integer b) {
6       this.a = a;
7       this.b = b;
8     }
9
10     //...
11 }
12
13 class FooBuilder {
14   private String a = ""; 
15   private Integer b = 0;
16
17   FooBuilder setA(String a) {
18     this.a = a;
19     return this;
20   }
21
22   FooBuilder setB(Integer b) {
23     this.b = b;
24     return this;
25   }
26
27   Foo build() {
28     return new Foo(a, b);
29   }
30 }
31
32 Foo foo = new FooBuilder().setA("a").build();

1void foo(String a, Integer b, Integer c) {
2    b = b != null ? b : 0;
3    c = c != null ? c : 0;
4    //...
5}
6
7foo("a", null, 2);

1void foo(Map<String, Object> parameters) {
2    String a = ""; 
3    Integer b = 0;
4    if (parameters.containsKey("a")) { 
5        if (!(parameters.get("a") instanceof Integer)) { 
6            throw new IllegalArgumentException("...");
7        }
8        a = (Integer)parameters.get("a");
9    }
10    if (parameters.containsKey("b")) { 
11        //... 
12    }
13    //...
14}
15
16foo(ImmutableMap.<String, Object>of(
17    "a", "a",
18    "b", 2, 
19    "d", "value"));