Demo: creating and resolving a merge conflict

Written Explanation

The plan

A conflict needs two branches that change the same line differently. The recipe:

1. Start from a clean main with a line that both branches will edit.
2. On a branch, change that line and commit.
3. On main, change the same line differently and commit.
4. Merge the branch into main.

We will use a monthly fee in Account.java. The sentence to remember: a conflict is two people deciding different things about the same line.

Step 0: A shared starting point

Check you are on main with nothing pending:

git status

Add one line to Account (below the balance field) and commit it, so that both branches start from it:

    private double monthlyFee = 5.0;

git commit -am "Add monthly fee field"

(-a stages changes to already tracked files, then commits. It does not add new files.)

Step 1: Change the line on a branch

git switch -c add-fee-waiver

Edit the line to private double monthlyFee = 0.0; (the fee is waived), then:

git commit -am "Waive monthly fee"

Step 2: Change the same line on main

git switch main

Edit the same line to private double monthlyFee = 7.5;, then:

git commit -am "Raise monthly fee to 7.5"

Both branches now contain a different decision about one line. The two lines of history have diverged, as in lesson L04.

Step 3: Merge and meet the conflict

git merge add-fee-waiver

Auto-merging src/Account.java
CONFLICT (content): Merge conflict in src/Account.java
Automatic merge failed; fix conflicts and then commit the result.

Git did not give up and did not lose anything; it paused. git status -s shows:

UU src/Account.java

UU means the file is unmerged, modified on both sides. Open the file:

public class Account {
    private final String owner;
    private double balance;
<<<<<<< HEAD
    private double monthlyFee = 7.5;
=======
    private double monthlyFee = 0.0;
>>>>>>> add-fee-waiver

    public Account(String owner, double balance) {

Above ======= is the current branch (main: 7.5). Below is add-fee-waiver (0.0). You can still stop here and run git merge --abort to go back to before the merge, if you want to start over.

Step 4: The planned mistake, commit with markers

A hurried developer sees the conflict, runs:

git add src/Account.java
git commit -m "Merge add-fee-waiver"

[main 5d9be22] Merge add-fee-waiver

Git accepted it. Git marks a conflict as resolved when you git add the file; it does not read the file to see whether the markers are gone. But the project no longer builds:

javac -d out src/*.java

src\Account.java:4: error: illegal start of type
<<<<<<< HEAD
^
src\Account.java:4: error: > expected
<<<<<<< HEAD
            ^

(The path separator appears as \ on Windows and / elsewhere. There are further error lines.) The compiler messages point at the first marker line. That is your clue: "illegal start of type" on a line starting with <<<<<<< means unresolved markers.

Step 5: Fix it, and a second trap

The quick-fix a beginner tries is to delete only the three marker lines. That leaves both versions of the line:

    private double monthlyFee = 7.5;
    private double monthlyFee = 0.0;

and the compiler says:

src\Account.java:5: error: variable monthlyFee is already defined in class Account

This shows what resolving means: you choose the final text. Both lines cannot stay. The team decides the fee is waived, so keep 0.0 and delete the 7.5 line. The file reads:

    private double monthlyFee = 0.0;

Check, then record the fix as a normal commit:

javac -d out src/*.java
git diff --stat
git commit -am "Resolve fee conflict: waive monthly fee, remove leftover markers"

 src/Account.java | 4 ----
 1 file changed, 4 deletions(-)

Confirm no markers remain anywhere:

git grep -n "<<<<<<<"

It prints nothing (exit status 1 means no match). The earlier bad merge commit stays in history, with the fix right after it. That is honest and good enough for this stage. Rewriting history to hide the slip is an advanced topic, and for commits that other people already have it can cause trouble, so we do not do it here.

Step 6: See the merge

git log --oneline --graph

* 115b825 Resolve fee conflict: waive monthly fee, remove leftover markers
*   5d9be22 Merge add-fee-waiver
|\  
| * ed81b2e Waive monthly fee
* | 7f31231 Raise monthly fee to 7.5
|/  
* 6683578 Add monthly fee field
* ab0fa5e Add deposit and withdraw
* 2948ea1 Add Main that creates one account
* 833d77b Add Account class with owner and balance

You can see the shared commit at the bottom, the two diverging commits, the merge commit that joins them, and the fix on top. You can now delete the merged branch:

git branch -d add-fee-waiver

The correct order, summarised

When you do it carefully the sequence is shorter, with no broken commit:

1. git merge branch (conflict reported).
2. git status (which files?).
3. Edit each file to the final text, with all markers removed.
4. Compile (and run tests, when you have them).
5. git add file.
6. git commit.