The parameter type of indexOf() isn't understandable,on page 107(Java OCA 8 Programmer I Study Guide

[Mushfiq Mammadov]

On page 107, first and second rows are:

int indexOf(char ch)
int indexOf(char ch, index fromIndex)

When I practice this code in Netbeans I saw that the parameter type of indexOf() is int.


Then I look through Oracle documentation:

public int indexOf(int ch)
public int indexOf(int ch, int fromIndex)

It is confusing for me, parameter type is int but we pass char type value as parameter?! When we pass int, return value always be “-1”. What happens?


When I note above question I saw another minor type. On the same page, second and fourth rows, second parameter type of method are incorrect:

int indexOf(char ch, index fromIndex)
int indexOf(String str, index fromIndex)

should be:

int indexOf(int ch, int fromIndex)
int indexOf(String str, int fromIndex)

[Roel De Nijs]

Mushfiq Mammadov wrote:When I note above question I saw another minor type. On the same page, second and fourth rows, second parameter type of method are incorrect:

True! That's a typo: index is of course not a valid (primitive) data type and should be replaced with int

[Roel De Nijs]

Mushfiq Mammadov wrote:It is confusing for me, parameter type is int but we pass char type value as parameter?! When we pass int, return value always be “-1”. What happens?

No, it doesn't return always -1. Let's have a look at this example:

String s = "Java rocks!";
System.out.printf("%d %d %d%n",s.indexOf(97),s.indexOf(98),s.indexOf(99));

Output: 1 -1 7

If you don't know what's happening behind the scenes, it looks like magic or even voodoo If you know what's happening, it's very easy (as always). Now let's look at this code snippet:

int i1 = 99;
char c1 = (char) i1;
System.out.printf("%d %c %b%n", i1, c1, i1 == c1); // 99 c true
char c2 = 'b';
int i2 = c2;
System.out.printf("%c %d %b%n", c2, i2, c2 == i2); // b 98 true
int i3 = 'a';
System.out.printf("%c %d%n", i3, i3); // a 97

Magic isn't it? What's really happening is very well explained in this (and following) posts. (Disclaimer: these posts are mine so probably a little biased )

[Mushfiq Mammadov]

Roel De Nijs wrote:
Let's have a look at this example:

String s = "Java rocks!";
System.out.printf("%d %d %d%n",s.indexOf(97),s.indexOf(98),s.indexOf(99));

Output: 1 -1 7

If you don't know what's happening behind the scenes, it looks like magic or even voodoo If you know what's happening, it's very easy (as always). Now let's look at this code snippet:

int i1 = 99;
char c1 = (char) i1;
System.out.printf("%d %c %b%n", i1, c1, i1 == c1); // 99 c true
char c2 = 'b';
int i2 = c2;
System.out.printf("%c %d %b%n", c2, i2, c2 == i2); // b 98 true
int i3 = 'a';
System.out.printf("%c %d%n", i3, i3); // a 97

Magic isn't it?

Wow! It is a very good example, Roel, I understand everything now I don't know we can initialize int as this: int i3 = 'a'; I also learn this. Thanks, Roel

[Jeanne Boyarsky]

Thanks Mushfiq. Added to the list.

The "index" parameter type is a typo. The "char" vs "int" was me simplifying the method signature as I did here. In hindsight, that wasn't a good decision as it didn't make things easier