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When does Common subexpression elimination apply? [closed]

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for (Pair p : pairs) {
double f = foo(p)
...
}
foo() performs a simple mathematical calculation as follows:
double foo(Pair p) {
return Math.cos(p.x) + Math.sin(p.y);
}
If all the items in pairs are the same, can the Java compiler optimise this using Common subexpression elimination? Or is there another optimisation that occurs? I am asking since I have found a significant time reduction when more values in pairs are the same.

Ternary operator in Java always returns null [closed]

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I don't have much experience with Java, but I use ternary operators in C#, and it's such a trivial thing that I don't think about it when I use it.
Recently I started a Spring Boot project, and I have something like this:
dto.setProfilePhoto(dao.getProfilePhoto() != null ? dao.getProfilePhoto() : "images/default-img.png");
but the profile photo always came back null.
Then I turned on debugger mode, and evaluated some expressions and this totally confused me...
Expression: boolean t = true ? true : false; ---> null
Expression: boolean t = 10 > 2 ? true : false; ---> null
Why do I get null values for these expressions?
Edit: Here are the getter and setter
public String getProfilePhoto() {
return ProfilePhoto;
}
public void setProfilePhoto(String profilePhoto) {
ProfilePhoto = profilePhoto;
}

You have not mentioned the implementation of dao.getProfilePhoto() e.g. it might be advancing to null in the second call. You can try it as follows:
String profilePhoto = dao.getProfilePhoto();
dto.setProfilePhoto(profilePhoto != null ? profilePhoto : "images/default-img.png");
It is not only efficient but also makes it agnostic of the implementation of dao.getProfilePhoto().

BufferedReader gives null when file is not in Documents [closed]

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I am working on a text editor software(kinda like notepad) and whenever I open files from Documents, correct data is displayed but at any other location Null is returned

This is because bff.readLine() is returning null. According to the documentation, it returns null if the end of the stream has been reached.
The previous check bff.readLine() != null doesn't help, because each call advances the reader. Try it like that:
String line;
while ((line = bff.readLine()) != null) {
sk += line + "\n";
}

Handling Invalid Character Constant [closed]

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I have been working on a data compression and decompression program in java. At some point in my code, I want to visit only the nodes with keys. The part of the code looks like this:
//visit only nodes with keys
if(n.alpha != '\0') {
System.out.println("{" + n.alpha + ":" + s + "}");
charToCode.put(n.alpha, s);
codeToChar.put(s, n.alpha);
}
'\0' gives me an invalid character constant. I need to know what is going on, and how I can remedy the situation. Thanks!

You're trying to represent and treat a String as a char
n.alpha != '\0' // single quotes denote a character
You need to use a String instead
n.alpha != "\0"
Then, because the equality operator generally shouldn't be used with Strings, and because you're incorrectly testing the equality between a String and a char, you need to rewrite it as
if(!Character.toString(n.alpha).equals("\0"))
Note the use of Character.toString(n.alpha) to convert the char n.alpha to a String.

Why didn't this give an error? [closed]

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I spent a long time on this. Why didn't it give some sort of error? I don't understand:
This java code snipette (I will put comments for code I omitted just for the example):
String[] sortOrder = { "Ascending","Descending" }
for ( String order : sortOrder ) {
if (sortOrder.equals("Ascending")) {
// code here to sort array in ascending
} else if ( sortOrder.equals( "Descending") ) {
// code here to sort descending
}
}

In the javadocs it states:
true if this object is the same as the obj argument; false otherwise.
The equals method is returning false instead of an error because your array is not equal to the
string value of "Ascending" and "Descending"