HI ! So I tried to add a sword using a custom class and and ItemTier class. Here's the relevant code :

/*package com.pandaw.magical_macrocosm.items;

import com.pandaw.magical_macrocosm.MagicalMacrocosm;
import com.pandaw.magical_macrocosm.init.ModItemTier;
import net.minecraft.item.Food;
import net.minecraft.item.IItemTier;
import net.minecraft.item.Item;
import net.minecraft.item.SwordItem;


*/

/* //How I tried to do at first
public class AdamasSword extends SwordItem {
    public AdamasSword() {
        super(new SwordItem(ModItemTier.ADAMAS,9,1.6f, (new Item.Properties())
                .group(MagicalMacrocosm.TAB))
        );
    }
}
*/

/* //What I found in the SwordItem class (tried to adjust it after, but here is the original version from SwordItem class)
public SwordItem(IItemTier tier, int attackDamageIn, float attackSpeedIn, Item.Properties builderIn) {
        super(tier, builderIn);
        this.attackDamage = (float)attackDamageIn + tier.getAttackDamage();
        Builder<Attribute, AttributeModifier> builder = ImmutableMultimap.builder();
        builder.put(Attributes.ATTACK_DAMAGE, new AttributeModifier(ATTACK_DAMAGE_MODIFIER, "Weapon modifier", (double)this.attackDamage, AttributeModifier.Operation.ADDITION));
        builder.put(Attributes.ATTACK_SPEED, new AttributeModifier(ATTACK_SPEED_MODIFIER, "Weapon modifier", (double)attackSpeedIn, AttributeModifier.Operation.ADDITION));
        this.attributeModifiers = builder.build();
        }
*/

package com.pandaw.magical_macrocosm.init;

import java.util.function.Supplier;
import net.minecraft.item.IItemTier;
import net.minecraft.item.crafting.Ingredient;
import net.minecraft.util.LazyValue;

public enum ModItemTier implements IItemTier {
    ADAMAS(4, 0, 6.0f, 9.0f, 16, () -> {
        return Ingredient.fromItems(ModItems.ADAMAS_DROP.get());
    });

    private final int harvestLevel;
    private final int maxUses;
    private final float efficiency;
    private final float attackDamage;
    private final int enchantability;
    private final LazyValue<Ingredient> repairMaterial;

    private ModItemTier(int harvestLevelIn, int maxUsesIn, float efficiencyIn, float attackDamageIn, int enchantabilityIn, Supplier<Ingredient> repairMaterialIn) {
        this.harvestLevel = harvestLevelIn;
        this.maxUses = maxUsesIn;
        this.efficiency = efficiencyIn;
        this.attackDamage = attackDamageIn;
        this.enchantability = enchantabilityIn;
        this.repairMaterial = new LazyValue<>(repairMaterialIn);
    }

    public int getMaxUses() {
        return this.maxUses;
    }

    public float getEfficiency() {
        return this.efficiency;
    }

    public float getAttackDamage() {
        return this.attackDamage;
    }

    public int getHarvestLevel() {
        return this.harvestLevel;
    }

    public int getEnchantability() {
        return this.enchantability;
    }

    public Ingredient getRepairMaterial() {
        return this.repairMaterial.getValue();
    }
}

Then I tried directly via registering : 

package com.pandaw.magical_macrocosm.init;

import com.pandaw.magical_macrocosm.MagicalMacrocosm;
import com.pandaw.magical_macrocosm.blocks.BlockItemBase;
import com.pandaw.magical_macrocosm.items.ItemBase;
import net.minecraft.item.Item;
import net.minecraft.item.ItemGroup;
import net.minecraft.item.ItemTier;
import net.minecraft.item.SwordItem;
import net.minecraftforge.fml.RegistryObject;
import net.minecraftforge.registries.DeferredRegister;
import net.minecraftforge.registries.ForgeRegistries;

public class ModItems {

    public static final DeferredRegister<Item> ITEMS = DeferredRegister.create(ForgeRegistries.ITEMS, MagicalMacrocosm.MOD_ID);

    //Items
    /*public static final RegistryObject<Item> ADAMAS_SWORD = ITEMS.register("adamas_sword", AdamasSword::new);*/ //When I tried the "custom class way"
    public static final RegistryObject<Item> ADAMAS_SWORD = ITEMS.register("adamas_sword", new SwordItem(ModItemTier.ADAMAS, 9, 1.6f, (new Item.Properties()).group(MagicalMacrocosm.TAB))); //The "registering way"

With first method, I have " 'SwordItem(net.minecraft.item.IItemTier, int, float, net.minecraft.item.Item.Properties)' in 'net.minecraft.item.SwordItem' cannot be applied to '(net.minecraft.item.SwordItem)' "

 

With the "registering" method; I have " Inferred type 'I' for type parameter 'I' is not within its bound; should extend 'net.minecraft.item.Item' "

 

Now, I know it may be simple, but I've just been stuck on that, and if someone is capable of explaining it to me, I'm open to know, thanks 😭

Edited August 25, 20214 yr by Pandaw

the register method requires a String and and Supplier
 

Spoiler

Quote

Supplier:

The Java Supplier interface is a functional interface that represents an function that supplies a value of some sorts. The Supplier interface can also be thought of as a factory interface. Here is an example implementation of the Java Supplier interface:

Supplier<Integer> supplier = () -> new Integer((int) (Math.random() * 1000D));

This Java Supplier implementation returns a new Integer instance with a random value between 0 and 1000.

quote from: http://tutorials.jenkov.com/java-functional-programming/functional-interfaces.html

 

Thank you !